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A species is defined as a group of individuals that, in nature, are able to mate and produce viable, fertile offspring.
- Explain the biological species concept
- Members of the same species are similar both in their external appearance and their internal physiology; the closer the relationship between two organisms, the more similar they will be in these features.
- Some species can look very dissimilar, such as two very different breeds of dogs, but can still mate and produce viable offspring, which signifies that they belong to the same species.
- Some species may look very similar externally, but can be dissimilar enough in their genetic makeup that they cannot produce viable offspring and are, therefore, different species.
- Mutations can occur in any cell of the body, but if a change does not occur in a sperm or egg cell, it cannot be passed on to the organism’s offspring.
- species: a group of organsms that, in nature, are capable of mating and producing viable, fertile offspring
- hybrid: offspring resulting from cross-breeding different entities, e.g. two different species or two purebred parent strains
- gene pool: the complete set of unique alleles that would be found by inspecting the genetic material of every living member of a species or population
Species and the Ability to Reproduce
A species is a group of individual organisms that interbreed and produce fertile, viable offspring. According to this definition, one species is distinguished from another when, in nature, it is not possible for matings between individuals from each species to produce fertile offspring.
Members of the same species share both external and internal characteristics which develop from their DNA. The closer relationship two organisms share, the more DNA they have in common, just like people and their families. People’s DNA is likely to be more like their father or mother’s DNA than their cousin’s or grandparent’s DNA. Organisms of the same species have the highest level of DNA alignment and, therefore, share characteristics and behaviors that lead to successful reproduction.
Species’ appearance can be misleading in suggesting an ability or inability to mate. For example, even though domestic dogs (Canis lupus familiaris) display phenotypic differences, such as size, build, and coat, most dogs can interbreed and produce viable puppies that can mature and sexually reproduce.
In other cases, individuals may appear similar although they are not members of the same species. For example, even though bald eagles (Haliaeetus leucocephalus) and African fish eagles (Haliaeetus vocifer) are both birds and eagles, each belongs to a separate species group. If humans were to artificially intervene and fertilize the egg of a bald eagle with the sperm of an African fish eagle and a chick did hatch, that offspring, called a hybrid (a cross between two species), would probably be infertile: unable to successfully reproduce after it reached maturity. Different species may have different genes that are active in development; therefore, it may not be possible to develop a viable offspring with two different sets of directions. Thus, even though hybridization may take place, the two species still remain separate.
Populations of species share a gene pool: a collection of all the variants of genes in the species. Again, the basis to any changes in a group or population of organisms must be genetic for this is the only way to share and pass on traits. When variations occur within a species, they can only be passed to the next generation along two main pathways: asexual reproduction or sexual reproduction. The change will be passed on asexually simply if the reproducing cell possesses the changed trait. For the changed trait to be passed on by sexual reproduction, a gamete, such as a sperm or egg cell, must possess the changed trait. In other words, sexually-reproducing organisms can experience several genetic changes in their body cells, but if these changes do not occur in a sperm or egg cell, the changed trait will never reach the next generation. Only heritable traits can evolve. Therefore, reproduction plays a paramount role for genetic change to take root in a population or species. In short, organisms must be able to reproduce with each other to pass new traits to offspring.
Biological species concept
The biological species concept gives an explanation of how species form (speciation). A biological species is a group of individuals that can breed together (panmixia). However, they cannot breed with other groups. In other words, the group is reproductively isolated from other groups.  
"The words 'reproductively isolated' are the key words of the biological species definition". Ernst Mayr.  p273
According to Ernst Mayr, a new species forms when an existing species splits. A similar idea had been suggested in the 19th century by Moritz Wagner.   Dobzhansky described the role of reproductive isolation in the formation of new species.  Once a species lives in two different areas, the geographical isolation makes breeding between the groups reduce or stop. Each group develops features which make breeding between them work less well. Eventually, each group becomes a 'good' biological species, because the two species do not reproduce with each other even when they are together.
This is still considered the most common reason for species splitting, and has the technical name of allopatric speciation. It is contrasted with sympatric speciation where speciation takes place even though all the members live in the same area. 
The species concept "problem" has pervaded for many years and will not be resolved anytime soon, if ever. The problem, of course, being that no two scientists will agree on universal definitions of what the darn things are! Taxonomist are exceptional argumentative and someone will undoubtedly disagree with everything in this article!
Species concepts were first defined based on morphological traits. Linneaus, being limited by technology at the time, used the "eyeball method" to study things - meaning he looked at them and described what he saw. This is formalized as the morphological or typological species concept (Cracraft, 2000 Mayr, 1996), and many biologists are just fine with this. It looks different, ergo it is and any distinguishing characters that could be observed, counted and measured were enough to define new species.
Characters are delimited by the practicing taxonomist and thus not all-inclusive of the whole organism. Morphological characters are typically those most easily observed, although the level of observation (i.e. from external features to cellular features) can have large effects on species identification and definition. For instance, Winston (1999) describes a case where closer observation of a western Atlantic species of the hermit crab hitch-hiking hydroid Hydractinia echinata, typically found off the coasts of Europe, resulted in the description of two additional species based on previously “hidden”, or non-scrutinized, morphological and ecological characters (Buss and Yund, 1989). Similar case are all too common. The closer we peer, the more we find.
The typological species concept idealized a species into an individual that represented a character or suite of characters that differentiated it from all other individuals. Thus, followers of this concept were forced to ignore population level variation. This plasticity of traits causes confusion and obscures the nature of a species particular adaptations. An extreme, but really fascinating, example comes from deep-sea hydrothermal vents off the coast of the Pacific northwest. A large polychaete tubeworm, Ridgeia piscesae, was originally described as two species due to two very unique morphotypes (see image below). Hydrothermal vent tubeworms are known to harbor symbiotic bacteria which use hydrogen sulfide as a chemical energy source, which is readily abundant coming out of the vents.
[caption align="aligncenter" caption="Ridgeia piscesae, from Carney et al. 2007. a) generalized body form, b) short-fat morphotype, c) long-skinny morphotype."][/caption]
Despite living within a stone throw of each other, populations of the "short-fat" and "long-skinny" tubeworm have completely different phenotypes. It was a few years later that with a suite of nuclear and mitochondrial genetic markers that it was realized that the two morphotypes were genetically indistinguishable. This phenotypic plasticity is the result of differential gene expression related to which environment the larvae settle in (Carney et al. 2007): active black smoker chimneys, which are characterized by higher hydrothermal flow, higher temperatures and greater sulfide concentrations or diffuse-flow hydrothermal fields. Even though the morphology is so different they were combined into one species.
Mayr (1942) brought the species concept from the individual level to the population level by defining species as discrete populations of individuals that are reproductively isolated, or unable to. A problem with this view of species is that data on inter-breeding are typically not known and museum specimens are often collected with disregard to such data (Wheeler, 1999). Mallet (1995) even went so as to call reproductive isolation a useless concept because it cannot be tested. Others have countered that all species concepts are inherently untestable by experimentation or observation (Coyne and Orr, 2004). Claims of biological species are often typological species in practice.
Mallet (1995) defined the genotypic cluster species concept to refute some of the pitfalls of the biological species concept and incorporate additional knowledge from genetics in terms of ‘identifiable genotypic clusters’ with no appreciable heterozygotes. Coyne and Orr (2004) argued that the genotypic clustering species concept focuses on identification of species and not the origin of species, is not conservative enough, and will over-recognize species in sympatry compared to the biological species concept. Furthermore, they argue that since the genotypic clustering concept is non-hierarchical, it doesn’t reflect the hierarchical nature of evolution and confuses polymorphic forms and Batesian mimics without introducing a reproductive criterion (Coyne and Orr, 2004).
While the biological species concept emphasizes isolating mechanisms which separate members of a species, the recognition (Patterson, 1985) and cohesion (Templeton, 1989) species concepts emphasize keeping members of a species cluster intact. The recognition species concept focuses on a shared fertilization system between individuals. Thus, it can only consider barriers to fertilization as modes of speciation. Coyne et al. (1988) considered this to be a subset of the biological species concept. Templeton (1989), on the other hand, contended that the advantage of the cohesion species concept was its emphasis on mechanisms that enforced gene flow between populations. This made it superior to the biological species concept in dealing with asexual and hybridized sympatric clusters that maintained their identities. Harrison (1998) brought a particular valid criticism to the approach of cohesion: “. life cycles and habitat associations have not been molded by selection for the purpose of ‘cohesion’.” (pg. 25). That is to say that selection appears to be a non-cohesive force by definition.
There are other concepts as well, Mayden (1997) lists 22-24 different conceptualizations and philosopher of science John Wilkins* lists 26. The above concepts view the species as the end-point of evolution, without considering the historical nature of the process of evolution. Hennig (1966) recognized this fact and argued for a temporal component to systematic theory which he termed phylogenetic systematics. Though many subsequent authors agree with Hennig in the use of a phylogenetic concept of species, several authors disagree on the particulars. This has led to differences in interpretation of what a species is and how species are related to one another. The Hennigian species concept incorporated the interbreeding model of a biological species concept, but with a historical component. This was modified by Willmann (1986) to specifically state that species are reproductively isolated and originate via a stem species branching off into two new species. The stem species, by definition, ceases to exist by way of either extinction of speciation. The latter point is important to proponents of this species concept because with dissolution of the stem species, monophyly (a species and all of its descendants) can be maintained.
Other authors have other interpretations of what a phylogenetic species concept is. The main concepts differ between whether species are viewed as irreducible clusters that are diagnostically distinct from other clusters (Wheeler and Nixon, 1990), as exclusive monophyletic units (de Queiroz and Donoghue, 1988), or as a group of organisms whose genes have more recently coalesced with each other relative to organisms outside that group and containing no exclusive group within it (Baum and Donoghue, 1995). At first glance, the species concept proposed by Baum and Donoghue (1995) appears to most accurately reflect evolutionary history. Upon closer inspection, it is nearly impossible to have complete knowledge of the evolutionary history of all genes in all the organisms in an analysis.
In practice, purveyors or this species concept have often used one or a few loci in delimiting species (Coyne and Orr, 2004). Shaw (2001) relaxed this extreme assumption to “greater than 50%”, meaning that a species is delimited if most of the genes have coalesced. While operationally useful, this definition may be just as arbitrary as using diagnostic morphological characters. Describing species as exclusive monophyletic units seeks to overcome such arbitrariness and potentially has the greatest power of all species concepts discussed here to represent a true phylogeny. But it is known that phylogenies based upon genes do not necessary reflect a species true phylogeny, which may never be known with certainty (Avise and Wollenberg, 1997).
Proponents of the evolutionary species concept claim theirs can be mos universally applied relative to all others. Wiley (1978) purported that a species concept must satisfy five criteria: universal validity, allow for testable hypotheses, include valid special case species definitions, specify what types of species origins are possible or not possible, and be “capable of dealing with species as spatial, temporal, genetic, epigenetic, ecological, physiological, phenetic, and behavioral entities” (pg. 18). Modifying an earlier concept from Simpson (1961), Wiley states: “A species is a single lineage of ancestral descendant populations of organisms which maintains its identity from other such lineages and which has its own evolutionary tendencies and historical fate.” Wiley’s modification removed the need for species to be changing, as originally defined by Simpson (1961). The criticisms levied against the evolutionary species concept appear to be more about lack of operational criteria to delimit species spatio-temporally (Wheeler and Meier, 2000).
An additional outcome of the species concept debate is the view that only populations are real and that species are artifacts (Brooks and McLennan, 1999). Darwin (1859) believed that species were arbitrary constructs of the taxonomist for convenience, while Mayr believed that species were real entities (Mayr, 1996). Levin (1979) championed the view that species are the empirical units of evolution and ecology, while evolutionary species concept supporters argued that if monophyletic groups are real then so are species (Wiley and Mayden, 2000b). Other interpretations span the range between arbitrary constructs and representing real natural entities. Furthermore, it seems as if every taxonomist is trying to find that one perfect species concept that works for all scenarios and types of organisms (Hey, 2001 Hey, 2006 Wheeler and Meier, 2000).
Several authors have advocated for pluralism, or the use of multiple species concepts (Mayden, 1997 Mayden, 1999). Different situations or questions may call for using different species concepts. Hey (2006) cautions against this though, stating it doesn’t help to settle anything regarding the species debate. Fitzhugh (2006) treads close to a pluralistic view species in his advocacy for a "requirement of total evidence" approach to systematics. This requirement suggests that any evidence relevant to the species question needs to be considered. Total evidence could include morphological character information, genetic characters, behavioral traits and more. While perhaps not setting out to satisfy multiple species concepts, the requirement for total evidence may do just that along the way.
As with many biologists studying biodiversity and other taxonomists, I feel unsatisfied by the current plethora of species concepts. Those attempting to be generally applied, such as the phylogenetic, Hennigian and evolutionary species concepts, tend to inflate biodiversity by elevating subspecies, or perhaps even distinct populations to species status. I’m unsure whether this inflation is due to the flexibility in the definitions, viewing species as lineages or clusters, or the taxonomic practice of the practitioners.
Those attempting to restrict the definition or discount evolutionary processes, like the biological species concept, tend to underestimate biodiversity. Additionally, the biological, recognition and cohesive species concepts cannot satisfactorily deal with asexual organisms and are unable to be applied broadly within only the animal kingdom. While reproductive isolation may be an important criterion for speciation to occur, other mechanisms are known such as hybridization, recombination, horizontal gene transfer (can occur between a eukaryote and prokaryote as shown in recent research: see Hotopp et al., 2007) to name but a few. Reproductive isolation may also be a product of speciation and not a causal mechanism (Mishler and Donoghue, 1982 Wiley and Mayden, 2000a).
As do the proponents of the evolutionary species concepts, I believe that species are real, are individuals and ancestral species do not need to become extinct during a speciation event. I view the act of formally describing a species as formulating a hypothesis about that species' unique suite of characters and the evolutionary history of the retention, loss or modification of those characters over time. I believe the evolutionary species comes closest to my views of what species are. I agree that species are entities of organisms that maintain its identity throughout time and space from other entities. This is a key factor to species being operationally and pragmatically useful. I understand this might not sit well with philosophers and some other evolutionary biologists. Some of the phylogenetic species concepts recognize too many species, whereas some of reproductive isolationist concepts ignore asexual and allopatric species. The latter is unacceptable and the former may give a misleading foundation for other areas of study (i.e. biodiversity ecology) to test hypotheses on.
* Though I haven't read the book, John Wilkins is an authority on species concepts and recently published Species: A History of an Idea, which promises to be informative and interesting!
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ABOUT THE AUTHOR(S)
Kevin has a M.Sc. degree in biology from Penn State, a B.Sc. in Evolution and Ecology from University of California, Davis, and has worked at as a researcher at several major marine science institutions. His broad academic research interests have encompassed population genetics, biodiversity, community ecology, food webs and systematics of invertebrates at deep-sea chemosynthetic environments and elsewhere. Kevin has described several new species of anemones and shrimp. He is now a freelance writer, independent scientist and science communications consultant living near the Baltic coast of Sweden in a small, idyllic village.
Kevin is also the assistant editor and webmaster for Deep Sea News, where he contributes articles on marine science. His award-winning writing has been appeared in Seed Magazine, The Open Lab: Best Writing on Science Blogs (2007, 2009, 2010), Discovery Channel, ScienceBlogs, and Environmental Law Review among others. He spends most of his time enjoying the company of his wife and two kids, hiking, supporting local breweries, raising awareness for open access, playing guitar and songwriting. You can read up more about Kevin and listen to his music at his homepage, where you can also view his CV and Résumé, and follow him twitter and Google +.
During the Age of Enlightenment (an era from the 1650s to the 1780s), concepts of monogenism and polygenism became popular, though they would only be systematized epistemologically during the 19th century. Monogenism contends that all races have a single origin, while polygenism is the idea that each race has a separate origin. Until the 18th century, the words "race" and "species" were interchangeable. 
François Bernier (1620–1688) was a French physician and traveller. In 1684 he published a brief essay dividing humanity into what he called "races", distinguishing individuals, and particularly women, by skin color and a few other physical traits. The article was published anonymously in the Journal des Savants, the earliest academic journal published in Europe, and titled "New Division of the Earth by the Different Species or 'Races' of Man that Inhabit It." 
In the essay he distinguished four different races: 1) The first race included populations from Europe, North Africa, the Middle East, India, south-east Asia, and the Americas, 2) the second race consisted of the sub-Saharan Africans, 3) the third race consisted of the east- and northeast Asians, and 4) the fourth race were Sámi people. The emphasis on different kinds of female beauty can be explained because the essay was the product of French Salon culture. Bernier emphasized that his novel classification was based on his personal experience as a traveler in different parts of the world. Bernier offered a distinction between essential genetic differences and accidental ones that depended on environmental factors. He also suggested that the latter criterion might be relevant to distinguish sub-types.  His biological classification of racial types never sought to go beyond physical traits, and he also accepted the role of climate and diet in explaining degrees of human diversity. Bernier had been the first to extend the concept of "species of man" to classify racially the entirety of humanity, but he did not establish a cultural hierarchy between the so-called 'races' that he had conceived. On the other hand he clearly placed white Europeans as the norm from which other 'races' deviated.  
The qualities which he attributed to each race were not strictly Eurocentric, because he thought that peoples of temperate Europe, the Americas and India, culturally very different, belonged to roughly the same racial group, and he explained the differences between the civilizations of India (his main area of expertise) and Europe through climate and institutional history. By contrast he emphasized the biological difference between Europeans and Africans, and made very negative comments towards the Sámi (Lapps) of the coldest climates of Northern Europe  and about Africans living at the Cape of Good Hope. He wrote for example "The 'Lappons' compose the 4th race. They are a small and short race with thick legs, wide shoulders, a short neck, and a face that I don't know how to describe, except that it's long, truly awful and seems reminiscent of a bears face. I've only ever seen them twice in Danzig, but according to the portraits I've seen and from what I've heard from a number of people they're ugly animals".  The significance of Bernier for the emergence of what Joan-Pau Rubiés call the "modern racial discourse" has been debated, with Siep Stuurman calling it the beginning of modern racial thought,  while Joan-Pau Rubiés think it is less significant if Bernier's entire view of humanity is taken into account. 
Robert Boyle vs. Henri de Boulainvilliers
An early scientist who studied race was Robert Boyle (1627–1691), an Anglo-Irish natural philosopher, chemist, physicist, and inventor. Boyle believed in what today is called 'monogenism', that is, that all races, no matter how diverse, came from the same source, Adam and Eve. He studied reported stories of parents' giving birth to different coloured albinos, so he concluded that Adam and Eve were originally white and that whites could give birth to different coloured races. Theories of Robert Hooke and Isaac Newton about color and light via optical dispersion in physics were also extended by Robert Boyle into discourses of polygenesis,  speculating that maybe these differences were due to "seminal impressions". However, Boyle's writings mention that at his time, for "European Eyes", beauty was not measured so much in colour, but in "stature, comely symmetry of the parts of the body, and good features in the face".  Various members of the scientific community rejected his views and described them as "disturbing" or "amusing". 
On the other hand, historian Henri de Boulainvilliers (1658–1722) divided the French as two races: (i) the aristocratic "French race" descended from the invader Germanic Franks, and (ii) the indigenous Gallo-Roman race (the political Third Estate populace). The Frankish aristocracy dominated the Gauls by innate right of conquest.
In his time, Henri de Boulainvilliers, a believer in the "right of conquest", did not understand "race" as biologically immutable, but as a contemporary cultural construct. [ citation needed ] His racialist account of French history was not entirely mythical: despite "supporting" hagiographies and epic poetry, such as The Song of Roland (La Chanson de Roland, c. 12th century), he sought scientific legitimation by basing his racialist distinction on the historical existence of genetically and linguistically distinguished Germanic and Latin-speaking peoples in France. His theory of race was distinct from the biological facts manipulated in 19th-century scientific racism [ citation needed ] (cf. Cultural relativism).
Richard Bradley (1688–1732) was an English naturalist. In his book "Philosophical Account of the Works of Nature" (1721), he claimed there to be "five sorts of men" based on their skin colour and other physical characteristics: white Europeans with beards white men in America without beards (meaning Native Americans) men with copper colour skin, small eyes and straight black hair Blacks with straight black hair and Blacks with curly hair. It has been speculated that his account inspired Linnaeus' later categorisation. 
The Scottish lawyer Henry Home, Lord Kames (1696–1782) was a polygenist he believed God had created different races on Earth in separate regions. In his 1734 book Sketches on the History of Man, Home claimed that the environment, climate, or state of society could not account for racial differences, so the races must have come from distinct, separate stocks. 
Carl Linnaeus (1707–1778), the Swedish physician, botanist, and zoologist, modified the established taxonomic bases of binomial nomenclature for fauna and flora, and also made a classification of humans into different subgroups. In the twelfth edition of Systema Naturae (1767), he labeled five  "varieties"   of human species. Each one was described as possessing the following physiognomic characteristics "varying by culture and place": 
- The Americanus: red, choleric, righteous black, straight, thick hair stubborn, zealous, free painting himself with red lines, and regulated by customs. 
- The Europeanus: white, sanguine, browny with abundant, long hair blue eyes gentle, acute, inventive covered with close vestments and governed by laws. 
- The Asiaticus: yellow, melancholic, stiff black hair, dark eyes severe, haughty, greedy covered with loose clothing and ruled by opinions. 
- The Afer or Africanus: black, phlegmatic, relaxed black, frizzled hair silky skin, flat nose, tumid lips females without shame mammary glands give milk abundantly crafty, sly, lazy, cunning, lustful, careless anoints himself with grease and governed by caprice. 
- The Monstrosus were mythologic humans which did not appear in the first editions of Systema Naturae. The sub-species included the "four-footed, mute, hairy" Homo feralis (Feral man) the animal-reared Juvenis lupinus hessensis (Hessian wolf boy), the Juvenis hannoveranus (Hannoverian boy), the Puella campanica (Wild-girl of Champagne), and the agile, but faint-hearted Homo monstrosus (Monstrous man): the Patagonian giant, the Dwarf of the Alps, and the monorchidKhoikhoi (Hottentot). In Amoenitates academicae (1763), Linnaeus presented the mythologicHomo anthropomorpha (Anthropomorphic man), humanoid creatures, such as the troglodyte, the satyr, the hydra, and the phoenix, incorrectly identified as simian creatures. 
There are disagreements about the basis for Linnaeus' human taxa. On the one hand, his harshest critics say the classification was not only ethnocentric but seemed to be based upon skin-color. Renato G Mazzolini have argued the skin-colour based classification at its core were a white/black polarity, and that Linnaeus thinking became paradigmatic for later racist thinking.  On the other hand, Quintyn (2010) points out that some authors believe the classification was based upon geographical distribution, being cartographically based, and not hierarchical.  In the opinion of Kenneth A.R. Kennedy (1976), Linnaeus certainly considered his own culture better, but his motives for classification of human varieties were not race-centered.  Paleontologist Stephen Jay Gould (1994) argued that the taxa was "not in the ranked order favored by most Europeans in the racist tradition", and that Linnaeus' division was influenced by the medical theory of humors which said that a person's temperament may be related to biological fluids.   In a 1997 essay, Gould added: "I don't mean to deny that Linnaeus held conventional beliefs about the superiority of his own European variety over others. nevertheless, and despite these implications, the overt geometry of Linnaeus' model is not linear or hierarchical." 
In a 2008 essay published by the Linnean Society of London, Marie-Christine Skuncke interpreted Linnaeus' statements as reflecting a view that "Europeans' superiority resides in "culture", and that the decisive factor in Linnaeus' taxa was "culture", not race. Thus, regarding this topic, they consider Linnaeus' view as merely "eurocentric", arguing that Linnaeus never called for racist action, and did not use the word "race", which was only introduced later "by his French opponent Buffon".  However, the anthropologist Ashley Montagu, in his book Man's Most Dangerous Myth: the Fallacy of Race, points out that Buffon, indeed "the enemy of all rigid classifications",  was diametrically opposed to such broad categories and did not use the word "race" to describe them. "It was quite clear, after reading Buffon, that he uses the word in no narrowly defined, but rather in a general sense,"  wrote Montagu, pointing out that Buffon did employ the French word la race, but as a collective term for whatever population he happened to be discussing at the time: for instance, "The Danish, Swedish, and Muscovite Laplanders, the inhabitants of Nova-Zembla, the Borandians, the Samoiedes, the Ostiacks of the old continent, the Greenlanders, and the savages to the north of the Esquimaux Indians, of the new continent, appear to be of one common race." 
Scholar Stanley A. Rice agrees that Linnaeus' classification was not meant to "imply a hierarchy of humanness or superiority"  although modern critics see that his classification was obviously stereotyped, and erroneous for having included anthropological, non-biological features such as customs or traditions.
John Hunter (1728–1793), a Scottish surgeon, said that originally the Negroid race was white at birth. He thought that over time because of the sun, the people turned dark skinned, or "black". Hunter also said that blisters and burns would likely turn white on a Negro, which he believed was evidence that their ancestors were originally white. 
Charles White (1728–1813), an English physician and surgeon, believed that races occupied different stations in the "Great Chain of Being", and he tried to scientifically prove that human races have distinct origins from each other. He believed that whites and Negroes were two different species. White was a believer in polygeny, the idea that different races had been created separately. His Account of the Regular Gradation in Man (1799) provided an empirical basis for this idea. White defended the theory of polygeny by rebutting French naturalist Georges-Louis Leclerc, Comte de Buffon's interfertility argument, which said that only the same species can interbreed. White pointed to species hybrids such as foxes, wolves, and jackals, which were separate groups that were still able to interbreed. For White, each race was a separate species, divinely created for its own geographical region. 
Buffon and Blumenbach
The French naturalist Georges-Louis Leclerc, Comte de Buffon (1707–1788) and the German anatomist Johann Blumenbach (1752–1840) were proponents of monogenism, the concept that all races have a single origin.  Buffon and Blumenbach believed a "degeneration theory" of the origins of racial difference.  Both said that Adam and Eve were white and that other races came about by degeneration owing to environmental factors, such as climate, disease, and diet.  According to this model, Negroid pigmentation arose because of the heat of the tropical sun, that cold wind caused the tawny colour of the Eskimos, and that the Chinese had fairer skins than the Tartars because the former kept mostly in towns and were protected from environmental factors.  Environmental factors, poverty, and hybridization could make races "degenerate" and differentiate them from the original white race by a process of "raciation".  Unusually, both Buffon and Blumenbach believed that the degeneration could be reversed if proper environmental control was taken, and that all contemporary forms of man could revert to the original white race. 
According to Blumenbach, there are five races, all belonging to a single species: Caucasian, Mongolian, Negroid, American, and the Malay race. Blumenbach said: "I have allotted the first place to the Caucasian for the reasons given below which make me esteem it the primeval one." 
Before James Hutton and the emergence of scientific geology, many believed the earth was only 6,000 years old. Buffon had conducted experiments with heated balls of iron which he believed were a model for the earth's core and concluded that the earth was 75,000 years old, but did not extend the time since Adam and the origin of humanity back more than 8,000 years – not much further than the 6,000 years of the prevailing Ussher chronology subscribed to by most of the monogenists.  Opponents of monogenism believed that it would have been difficult for races to change markedly in such a short period of time. 
Benjamin Rush (1745–1813), a Founding Father of the United States and a physician, proposed that being black was a hereditary skin disease, which he called "negroidism", and that it could be cured. Rush believed non-whites were really white underneath but they were stricken with a non-contagious form of leprosy which darkened their skin color. Rush drew the conclusion that "whites should not tyrannize over [blacks], for their disease should entitle them to a double portion of humanity. However, by the same token, whites should not intermarry with them, for this would tend to infect posterity with the 'disorder'. attempts must be made to cure the disease". 
Christoph Meiners (1747–1810) was a German polygenist and believed that each race had a separate origin. Meiner studied the physical, mental and moral characteristics of each race, and built a race hierarchy based on his findings. Meiners split mankind into two divisions, which he labelled the "beautiful white race" and the "ugly black race". In Meiners's book The Outline of History of Mankind, he said that a main characteristic of race is either beauty or ugliness. He thought only the white race to be beautiful. He considered ugly races to be inferior, immoral and animal-like. He said that the dark, ugly peoples were distinct from the white, beautiful peoples by their "sad" lack of virtue and their "terrible vices".  According to Meiners, [ citation needed ]
The more intelligent and noble people are by nature, the more adaptable, sensitive, delicate, and soft is their body on the other hand, the less they possess the capacity and disposition towards virtue, the more they lack adaptability and not only that, but the less sensitive are their bodies, the more can they tolerate extreme pain or the rapid alteration of heat and cold when they are exposed to illnesses, the more rapid their recovery from wounds that would be fatal for more sensitive peoples, and the more they can partake of the worst and most indigestible foods . without noticeable ill effects.
Meiners said the Negro felt less pain than any other race and lacked in emotions. Meiners wrote that the Negro had thick nerves and thus was not sensitive like the other races. He went as far as to say that the Negro has "no human, barely any animal, feeling". He described a story where a Negro was condemned to death by being burned alive. Halfway through the burning, the Negro asked to smoke a pipe and smoked it like nothing was happening while he continued to be burned alive. Meiners studied the anatomy of the Negro and came to the conclusion that Negroes have bigger teeth and jaws than any other race, as Negroes are all carnivores. Meiners claimed the skull of the Negro was larger but the brain of the Negro was smaller than any other race. Meiners claimed the Negro was the most unhealthy race on Earth because of its poor diet, mode of living and lack of morals. 
Meiners also claimed the "Americans" were an inferior stock of people. He said they could not adapt to different climates, types of food, or modes of life, and that when exposed to such new conditions, they lapse into a "deadly melancholy". Meiners studied the diet of the Americans and said they fed off any kind of "foul offal". He thought they consumed very much alcohol. He believed their skulls were so thick that the blades of Spanish swords shattered on them. Meiners also claimed the skin of an American is thicker than that of an ox. 
Meiners wrote that the noblest race was the Celts. They were able to conquer various parts of the world, they were more sensitive to heat and cold, and their delicacy is shown by the way they are selective about what they eat. Meiners claimed that Slavs are an inferior race, "less sensitive and content with eating rough food". He described stories of Slavs allegedly eating poisonous fungi without coming to any harm. He claimed that their medical techniques were also backward: he used as an example their heating sick people in ovens, then making them roll in the snow. 
In Meiners's large work entitled Researches on the Variations in Human Nature (1815), he studied also the sexology of each race. He claimed that the African Negroids have unduly strong and perverted sex drives, whilst only the white Europeans have it just right.
Thomas Jefferson (1743–1826) was an American politician, scientist,   and slave owner. His contributions to scientific racism have been noted by many historians, scientists and scholars. According to an article published in the McGill Journal of Medicine: "One of the most influential pre-Darwinian racial theorists, Jefferson's call for science to determine the obvious "inferiority" of African Americans is an extremely important stage in the evolution of scientific racism."  The historian Paul Finkelman described Jefferson in The New York Times as follows: "A scientist, Jefferson nevertheless speculated that blackness might come "from the color of the blood" and concluded that blacks were "inferior to the whites in the endowments of body and mind."  In his "Notes on the State of Virginia" Jefferson described black people as follows: 
They seem to require less sleep. A black, after hard labor through the day, will be induced by the slightest amusements to sit up till midnight, or later, though knowing he must be out with the first dawn of the morning. They are at least as brave, and more adventuresome. But this may perhaps proceed from a want of forethought, which prevents their seeing a danger till it be present. When present, they do not go through it with more coolness or steadiness than the whites. They are more ardent after their female: but love seems with them to be more an eager desire, than a tender delicate mixture of sentiment and sensation. Their griefs are transient. Those numberless afflictions, which render it doubtful whether heaven has given life to us in mercy or in wrath, are less felt, and sooner forgotten with them. In general, their existence appears to participate more of sensation than reflection. Comparing them by their faculties of memory, reason, and imagination, it appears to me, that in memory they are equal to the whites in reason much inferior, as I think one [black] could scarcely be found capable of tracing and comprehending the investigations of Euclid and that in imagination they are dull, tasteless, and anomalous. I advance it therefore as a suspicion only, that the blacks, whether originally a distinct race, or made distinct by time and circumstances, are inferior to the whites in the endowments both of body and mind.
However, by 1791, Jefferson had to reassess his earlier suspicions of whether blacks were capable of intelligence when he was presented with a letter and almanac from Benjamin Banneker, an educated black mathematician. Delighted to have discovered scientific proof for the existence of black intelligence, Jefferson wrote to Banneker: 
No body wishes more than I do to see such proofs as you exhibit, that nature has given to our black brethren, talents equal to those of the other colors of men, & that the appearance of a want of them is owing merely to the degraded condition of their existence both in Africa & America. I can add with truth that no body wishes more ardently to see a good system commenced for raising the condition both of their body & mind to what it ought to be, as fast as the imbecility of their present existence, and other circumstance which cannot be neglected, will admit.
Samuel Stanhope Smith
Samuel Stanhope Smith (1751–1819) was an American Presbyterian minister and author of Essay on the Causes of Variety of Complexion and Figure in the Human Species in 1787. Smith claimed that Negro pigmentation was nothing more than a huge freckle that covered the whole body as a result of an oversupply of bile, which was caused by tropical climates. 
Racial studies by Georges Cuvier (1769–1832), the French naturalist and zoologist, influenced scientific polygenism and scientific racism. Cuvier believed there were three distinct races: the Caucasian (white), Mongolian (yellow) and the Ethiopian (black). He rated each for the beauty or ugliness of the skull and quality of their civilizations. Cuvier wrote about Caucasians: "The white race, with oval face, straight hair and nose, to which the civilised people of Europe belong and which appear to us the most beautiful of all, is also superior to others by its genius, courage and activity". 
Regarding Negroes, Cuvier wrote: 
The Negro race . is marked by black complexion, crisped or woolly hair, compressed cranium and a flat nose. The projection of the lower parts of the face, and the thick lips, evidently approximate it to the monkey tribe: the hordes of which it consists have always remained in the most complete state of barbarism.
He thought Adam and Eve were Caucasian and hence the original race of mankind. The other two races arose by survivors' escaping in different directions after a major catastrophe hit the earth 5,000 years ago. He theorized that the survivors lived in complete isolation from each other and developed separately.  
One of Cuvier's pupils, Friedrich Tiedemann, was one of the first to make a scientific contestation of racism. He argued based on craniometric and brain measurements taken by him from Europeans and black people from different parts of the world that the then-common European belief that Negroes have smaller brains, and are thus intellectually inferior, is scientifically unfounded and based merely on the prejudice of travellers and explorers. 
The German philosopher Arthur Schopenhauer (1788–1860) attributed civilizational primacy to the white races, who gained sensitivity and intelligence via the refinement caused by living in the rigorous Northern climate: 
The highest civilization and culture, apart from the ancient Hindus and Egyptians, are found exclusively among the white races and even with many dark peoples, the ruling caste, or race, is fairer in colour than the rest, and has, therefore, evidently immigrated, for example, the Brahmins, the Inca, and the rulers of the South Sea Islands. All this is due to the fact that necessity is the mother of invention, because those tribes that emigrated early to the north, and there gradually became white, had to develop all their intellectual powers, and invent and perfect all the arts in their struggle with need, want, and misery, which, in their many forms, were brought about by the climate. This they had to do in order to make up for the parsimony of nature, and out of it all came their high civilization.
Franz Ignaz Pruner
Franz Ignaz Pruner (1808–1882) was a medical doctor who studied the racial structure of Negroes in Egypt. In a book which he wrote in 1846 he claimed that Negro blood had a negative influence on the Egyptian moral character. He published a monograph on Negroes in 1861. He claimed that the main feature of the Negro's skeleton is prognathism, which he claimed was the Negro's relation to the ape. He also claimed that Negroes had brains very similar to those of apes and that Negroes have a shortened big toe, a characteristic, he said, that connected Negroes closely to apes. 
The scientific classification established by Carl Linnaeus is requisite to any human racial classification scheme. In the 19th century, unilineal evolution, or classical social evolution, was a conflation of competing sociologic and anthropologic theories proposing that Western European culture was the acme of human socio-cultural evolution. The proposal that social status is unilineal—from primitive to civilized, from agricultural to industrial—became popular among philosophers, including Friedrich Hegel, Immanuel Kant, and Auguste Comte. The Christian Bible was interpreted to sanction slavery and from the 1820s to the 1850s was often used in the antebellum Southern United States, by writers such as the Rev. Richard Furman and Thomas R. Cobb, to enforce the idea that Negroes had been created inferior, and thus suited to slavery. 
Arthur de Gobineau
The French aristocrat and writer Arthur de Gobineau (1816–1882), is best known for his book An Essay on the Inequality of the Human Races (1853–55) which proposed three human races (black, white and yellow) were natural barriers and claimed that race mixing would lead to the collapse of culture and civilization. He claimed that "The white race originally possessed the monopoly of beauty, intelligence and strength" and that any positive accomplishments or thinking of blacks and Asians were due to an admixture with whites. His works were praised by many white supremacist American pro-slavery thinkers such as Josiah C. Nott and Henry Hotze.
Gobineau believed that the different races originated in different areas, the white race had originated somewhere in Siberia, the Asians in the Americas and the blacks in Africa. He believed that the white race was superior, writing:
I will not wait for the friends of equality to show me such and such passages in books written by missionaries or sea captains, who declare some Wolof is a fine carpenter, some Hottentot a good servant, that a Kaffir dances and plays the violin, that some Bambara knows arithmetic… Let us leave aside these puerilities and compare together not men, but groups. 
Gobineau later used the term "Aryans" to describe the Germanic peoples (la race germanique). 
Gobineau's works were also influential to the Nazi Party, which published his works in German. They played a key role in the master race theory of Nazism.
Another polygenist evolutionist was Carl Vogt (1817–1895) who believed that the Negro race was related to the ape. He wrote the white race was a separate species to Negroes. In Chapter VII of his Lectures of Man (1864) he compared the Negro to the white race whom he described as "two extreme human types". The difference between them, he claimed are greater than those between two species of ape and this proves that Negroes are a separate species from the whites. 
Charles Darwin's views on race have been a topic of much discussion and debate. According to Jackson and Weidman, Darwin was a moderate in the 19th century debates about race. "He was not a confirmed racist — he was a staunch abolitionist, for example — but he did think that there were distinct races that could be ranked in a hierarchy." 
Darwin's influential 1859 book On the Origin of Species did not discuss human origins. The extended wording on the title page, which adds by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life, uses the general terminology of biological races as an alternative for "varieties" and does not carry the modern connotation of human races. In The Descent of Man, and Selection in Relation to Sex (1871), Darwin examined the question of "Arguments in favour of, and opposed to, ranking the so-called races of man as distinct species" and reported no racial distinctions that would indicate that human races are discrete species.  
The historian Richard Hofstadter wrote, "Although Darwinism was not the primary source of the belligerent ideology and dogmatic racism of the late nineteenth century, it did become a new instrument in the hands of the theorists of race and struggle. The Darwinist mood sustained the belief in Anglo-Saxon racial superiority which obsessed many American thinkers in the latter half of the nineteenth century. The measure of world domination already achieved by the 'race' seemed to prove it the fittest."  According to the historian Gertrude Himmelfarb, "The subtitle of [The Origin of Species] made a convenient motto for racists: 'The Preservation of Favoured Races in the Struggle for Life.' Darwin, of course, took 'races' to mean varieties or species but it was no violation of his meaning to extend it to human races. Darwin himself, in spite of his aversion to slavery, was not averse to the idea that some races were more fit than others." 
On the other hand, Robert Bannister defended Darwin on the issue of race, writing that "Upon closer inspection, the case against Darwin himself quickly unravels. An ardent opponent of slavery, he consistently opposed the oppression of nonwhites. Although by modern standards The Descent of Man is frustratingly inconclusive on the critical issues of human equality, it was a model of moderation and scientific caution in the context of midcentury racism." 
Herbert Hope Risley
As an exponent of "race science", colonial administrator Herbert Hope Risley (1851–1911) used the ratio of the width of a nose to its height to divide Indian people into Aryan and Dravidian races, as well as seven castes.  
Like most of Darwin's supporters, [ citation needed ] Ernst Haeckel (1834–1919) put forward a doctrine of evolutionary polygenism based on the ideas of the linguist and polygenist August Schleicher, in which several different language groups had arisen separately from speechless prehuman Urmenschen (German for "original humans"), which themselves had evolved from simian ancestors. These separate languages had completed the transition from animals to man, and, under the influence of each main branch of languages, humans had evolved as separate species, which could be subdivided into races. Haeckel divided human beings into ten races, of which the Caucasian was the highest and the primitives were doomed to extinction.  Haeckel was also an advocate of the out of Asia theory by writing that the origin of humanity was to be found in Asia he believed that Hindustan (South Asia) was the actual location where the first humans had evolved. Haeckel argued that humans were closely related to the primates of Southeast Asia and rejected Darwin's hypothesis of Africa.  
Haeckel also wrote that Negroes have stronger and more freely movable toes than any other race which is evidence that Negroes are related to apes because when apes stop climbing in trees they hold on to the trees with their toes. Haeckel compared Negroes to "four-handed" apes. Haeckel also believed Negroes were savages and that whites were the most civilised. 
Nationalism of Lapouge and Herder
At the 19th century's end, scientific racism conflated Greco-Roman eugenicism with Francis Galton's concept of voluntary eugenics to produce a form of coercive, anti-immigrant government programs influenced by other socio-political discourses and events. Such institutional racism was effected via Phrenology, telling character from physiognomy craniometric skull and skeleton studies thus skulls and skeletons of black people and other colored volk, were displayed between apes and white men.
In 1906, Ota Benga, a Pygmy, was displayed as the "Missing Link", in the Bronx Zoo, New York City, alongside apes and animals. The most influential theorists included the anthropologist Georges Vacher de Lapouge (1854–1936) who proposed "anthroposociology" and Johann Gottfried Herder (1744–1803), who applied "race" to nationalist theory, thereby developing the first conception of ethnic nationalism. In 1882, Ernest Renan contradicted Herder with a nationalism based upon the "will to live together", not founded upon ethnic or racial prerequisites (see Civic nationalism). Scientific racist discourse posited the historical existence of "national races" such as the Deutsche Volk in Germany, and the "French race" being a branch of the basal "Aryan race" extant for millennia, to advocate for geopolitical borders parallel to the racial ones.
Craniometry and physical anthropology
The Dutch scholar Pieter Camper (1722–89), an early craniometric theoretician, used "craniometry" (interior skull-volume measurement) to scientifically justify racial differences. In 1770, he conceived of the facial angle to measure intelligence among species of men. The facial angle was formed by drawing two lines: a horizontal line from nostril to ear and a vertical line from the upper-jawbone prominence to the forehead prominence. Camper's craniometry reported that antique statues (the Greco-Roman ideal) had a 90-degree facial angle, whites an 80-degree angle, blacks a 70-degree angle, and the orangutan a 58-degree facial angle—thus he established a racist biological hierarchy for mankind, per the Decadent conception of history. Such scientific racist researches were continued by the naturalist Étienne Geoffroy Saint-Hilaire (1772–1844) and the anthropologist Paul Broca (1824–1880).
Samuel George Morton
In the 19th century, an early American physical anthropologist, physician and polygenist Samuel George Morton (1799–1851), collected human skulls from worldwide, and attempted a logical classification scheme. Influenced by contemporary racialist theory, Dr Morton said he could judge racial intellectual capacity by measuring the interior cranial capacity, hence a large skull denoted a large brain, thus high intellectual capacity. Conversely, a small skull denoted a small brain, thus low intellectual capacity superior and inferior established. After inspecting three mummies from ancient Egyptian catacombs, Morton concluded that Caucasians and Negroes were already distinct three thousand years ago. Since interpretations of the bible indicated that Noah's Ark had washed up on Mount Ararat only a thousand years earlier, Morton claimed that Noah's sons could not possibly account for every race on earth. According to Morton's theory of polygenesis, races have been separate since the start. 
In Morton's Crania Americana, his claims were based on Craniometry data, that the Caucasians had the biggest brains, averaging 87 cubic inches, Native Americans were in the middle with an average of 82 cubic inches and Negroes had the smallest brains with an average of 78 cubic inches. 
In The Mismeasure of Man (1981), the evolutionary biologist and historian of science Stephen Jay Gould argued that Samuel Morton had falsified the craniometric data, perhaps inadvertently over-packing some skulls, to so produce results that would legitimize the racist presumptions he was attempting to prove. A subsequent study by the anthropologist John Michael found Morton's original data to be more accurate than Gould describes, concluding that "[c]ontrary to Gould's interpretation. Morton's research was conducted with integrity".  Jason Lewis and colleagues reached similar conclusions as Michael in their reanalysis of Morton's skull collection however, they depart from Morton's racist conclusions by adding that "studies have demonstrated that modern human variation is generally continuous, rather than discrete or "racial", and that most variation in modern humans is within, rather than between, populations". 
In 1873, Paul Broca, founder of the Anthropological Society of Paris (1859), found the same pattern of measures—that Crania Americana reported—by weighing specimen brains at autopsy. Other historical studies, proposing a black race–white race, intelligence–brain size difference, include those by Bean (1906), Mall (1909), Pearl (1934), and Vint (1934).
After the War of the Pacific (1879–83) there was a rise of racial and national superiority ideas among the Chilean ruling class.  In his 1918 book physician Nicolás Palacios argued for the existence of Chilean race and its superiority when compared to neighboring peoples. He thought Chileans were a mix of two martial races: the indigenous Mapuches and the Visigoths of Spain, who descended ultimately from Götaland in Sweden. Palacios argued on medical grounds against immigration to Chile from southern Europe claiming that Mestizos who are of south European stock lack "cerebral control" and are a social burden. 
Monogenism and polygenism
Samuel Morton's followers, especially Dr Josiah C. Nott (1804–1873) and George Gliddon (1809–1857), extended Dr Morton's ideas in Types of Mankind (1854), claiming that Morton's findings supported the notion of polygenism (mankind has discrete genetic ancestries the races are evolutionarily unrelated), which is a predecessor of the modern human multiregional origin hypothesis. Moreover, Morton himself had been reluctant to espouse polygenism, because it theologically challenged the Christian creation myth espoused in the Bible.
Later, in The Descent of Man (1871), Charles Darwin proposed the single-origin hypothesis, i.e., monogenism—mankind has a common genetic ancestry, the races are related, opposing everything that the polygenism of Nott and Gliddon proposed.
One of the first typologies used to classify various human races was invented by Georges Vacher de Lapouge (1854–1936), a theoretician of eugenics, who published in 1899 L'Aryen et son rôle social (1899 – "The Aryan and his social role"). In this book, he classified humanity into various, hierarchized races, spanning from the "Aryan white race, dolichocephalic", to the "brachycephalic", "mediocre and inert" race, best represented by Southern European, Catholic peasants".  Between these, Vacher de Lapouge identified the "Homo europaeus" (Teutonic, Protestant, etc.), the "Homo alpinus" (Auvergnat, Turkish, etc.), and finally the "Homo mediterraneus" (Neapolitan, Andalus, etc.) Jews were brachycephalic like the Aryans, according to Lapouge but exactly for this reason he considered them to be dangerous they were the only group, he thought, threatening to displace the Aryan aristocracy.  Vacher de Lapouge became one of the leading inspirators of Nazi antisemitism and Nazi racist ideology. 
Vacher de Lapouge's classification was mirrored in William Z. Ripley in The Races of Europe (1899), a book which had a large influence on American white supremacism. Ripley even made a map of Europe according to the alleged cephalic index of its inhabitants. He was an important influence of the American eugenist Madison Grant.
Furthermore, according to John Efron of Indiana University, the late 19th century also witnessed "the scientizing of anti-Jewish prejudice", stigmatizing Jews with male menstruation, pathological hysteria, and nymphomania.   At the same time, several Jews, such as Joseph Jacobs or Samuel Weissenberg, also endorsed the same pseudoscientific theories, convinced that the Jews formed a distinct race.   Chaim Zhitlovsky also attempted to define Yiddishkayt (Ashkenazi Jewishness) by turning to contemporary racial theory. 
Joseph Deniker (1852–1918) was one of William Z. Ripley's principal opponents whereas Ripley maintained, as did Vacher de Lapouge, that the European populace comprised three races, Joseph Deniker proposed that the European populace comprised ten races (six primary and four sub-races). Furthermore, he proposed that the concept of "race" was ambiguous, and in its stead proposed the compound word "ethnic group", which later prominently featured in the works of Julian Huxley and Alfred C. Haddon. Moreover, Ripley argued that Deniker's "race" idea should be denoted a "type", because it was less biologically rigid than most racial classifications.
Joseph Deniker's contribution to racist theory was La Race nordique (the Nordic race), a generic, racial-stock descriptor, which the American eugenicist Madison Grant (1865–1937) presented as the white racial engine of world civilization. Having adopted Ripley's three-race European populace model, but disliking the "Teuton" race name, he transliterated la race nordique into "The Nordic race", the acme of the concocted racial hierarchy, based upon his racial classification theory, popular in the 1910s and 1920s.
State Institute for Racial Biology (Swedish: Statens Institut för Rasbiologi) and its director Herman Lundborg in Sweden were active in racist research. Furthermore, much of early research on Ural-Altaic languages was coloured by attempts at justifying the view that European peoples east of Sweden were Asian and thus of inferior race, justifying colonialism, eugenics and racial hygiene. [ citation needed ] The book The Passing of the Great Race (Or, The Racial Basis of European History) by American eugenicist, lawyer, and amateur anthropologist Madison Grant was published in 1916. Though influential, the book was largely ignored when it first appeared, and it went through several revisions and editions. Nevertheless, the book was used by people who advocated restricted immigration as justification for what became known as scientific racism. 
Justification of slavery in the United States
In the United States, scientific racism justified Black African slavery to assuage moral opposition to the Atlantic slave trade. Alexander Thomas and Samuell Sillen described black men as uniquely fitted for bondage, because of their "primitive psychological organization".  In 1851, in antebellum Louisiana, the physician Samuel A. Cartwright (1793–1863) wrote of slave escape attempts as "drapetomania", a treatable mental illness, that "with proper medical advice, strictly followed, this troublesome practice that many Negroes have of running away can be almost entirely prevented". The term drapetomania (mania of the runaway slave) derives from the Greek δραπέτης (drapetes, "a runaway [slave]") and μανία (mania, "madness, frenzy")  Cartwright also described dysaesthesia aethiopica, called "rascality" by overseers. The 1840 United States Census claimed that Northern, free blacks suffered mental illness at higher rates than did their Southern, enslaved counterparts. Though the census was later found to have been severely flawed by the American Statistical Association, it became a political weapon against abolitionists. Southern slavers concluded that escaping Negroes were suffering from "mental disorders". 
At the time of the American Civil War (1861–65), the matter of miscegenation prompted studies of ostensible physiological differences between Caucasians and Negroes. Early anthropologists, such as Josiah Clark Nott, George Robins Gliddon, Robert Knox, and Samuel George Morton, aimed to scientifically prove that Negroes were a human species different from the white people that the rulers of Ancient Egypt were not African and that mixed-race offspring (the product of miscegenation) tended to physical weakness and infertility. After the Civil War, Southern (Confederacy) physicians wrote textbooks of scientific racism based upon studies claiming that black freemen (ex-slaves) were becoming extinct, because they were inadequate to the demands of being a free man—implying that black people benefited from enslavement.
In Medical Apartheid, Harriet A. Washington noted the prevalence of two different views on blacks in the 19th century: the belief that they were inferior and "riddled with imperfections from head to toe", and the idea that they didn't know true pain and suffering because of their primitive nervous systems (and that slavery was therefore justifiable). Washington noted the failure of scientists to accept the inconsistency between these two viewpoints, writing that "in the eighteenth and nineteenth centuries, scientific racism was simply science, and it was promulgated by the very best minds at the most prestigious institutions of the nation. Other, more logical medical theories stressed the equality of Africans and laid poor black health at the feet of their abusers, but these never enjoyed the appeal of the medical philosophy that justified slavery and, along with it, our nation's profitable way of life." 
Even after the end of the Civil War, some scientists continued to justify the institution of slavery by citing the effect of topography and climate on racial development. Nathaniel Shaler, a prominent geologist at Harvard University from 1869-1906, published the book Man and the Earth in 1905 describing the physical geography of different continents and linking these geologic settings to the intelligence and strength of human races that inhabited these spaces. Shaler argued that North American climate and geology was ideally suited for the institution of slavery. 
South African apartheid
Scientific racism played a role in establishing apartheid in South Africa. In South Africa, white scientists, like Dudly Kidd, who published The essential Kafir in 1904, sought to "understand the African mind". They believed that the cultural differences between whites and blacks in South Africa might be caused by physiological differences in the brain. Rather than suggesting that Africans were "overgrown children", as early white explorers had, Kidd believed that Africans were "misgrown with a vengeance". He described Africans as at once "hopelessly deficient", yet "very shrewd". 
The Carnegie Commission on the Poor White Problem in South Africa played a key role in establishing apartheid in South Africa. According to one memorandum sent to Frederick Keppel, then president of the Carnegie Corporation, there was "little doubt that if the natives were given full economic opportunity, the more competent among them would soon outstrip the less competent whites".  Keppel's support for the project of creating the report was motivated by his concern with the maintenance of existing racial boundaries.  The preoccupation of the Carnegie Corporation with the so-called poor white problem in South Africa was at least in part the outcome of similar misgivings about the state of poor whites in the southern United States. 
The report was five volumes in length.  Around the start of the 20th century, white Americans, and whites elsewhere in the world, felt uneasy because poverty and economic depression seemed to strike people regardless of race. 
Though the ground work for apartheid began earlier, the report provided support for this central idea of black inferiority. This was used to justify racial segregation and discrimination  in the following decades.  The report expressed fear about the loss of white racial pride, and in particular pointed to the danger that the poor white would not be able to resist the process of "Africanisation". 
Although scientific racism played a role in justifying and supporting institutional racism in South Africa, it was not as important in South Africa as it has been in Europe and the United States. This was due in part to the "poor white problem", which raised serious questions for supremacists about white racial superiority.  Since poor whites were found to be in the same situation as natives in the African environment, the idea that intrinsic white superiority could overcome any environment did not seem to hold. As such, scientific justifications for racism were not as useful in South Africa. 
Stephen Jay Gould described Madison Grant's The Passing of the Great Race (1916) as "the most influential tract of American scientific racism." In the 1920s–30s, the German racial hygiene movement embraced Grant's Nordic theory. Alfred Ploetz (1860–1940) coined the term Rassenhygiene in Racial Hygiene Basics (1895), and founded the German Society for Racial Hygiene in 1905. The movement advocated selective breeding, compulsory sterilization, and a close alignment of public health with eugenics.
Racial hygiene was historically tied to traditional notions of public health, but with emphasis on heredity—what philosopher and historian Michel Foucault has called state racism. In 1869, Francis Galton (1822–1911) proposed the first social measures meant to preserve or enhance biological characteristics, and later coined the term "eugenics". Galton, a statistician, introduced correlation and regression analysis and discovered regression toward the mean. He was also the first to study human differences and inheritance of intelligence with statistical methods. He introduced the use of questionnaires and surveys to collect data on population sets, which he needed for genealogical and biographical works and for anthropometric studies. Galton also founded psychometrics, the science of measuring mental faculties, and differential psychology, a branch of psychology concerned with psychological differences between people rather than common traits.
Like scientific racism, eugenics grew popular in the early 20th century, and both ideas influenced Nazi racial policies and Nazi eugenics. In 1901, Galton, Karl Pearson (1857–1936) and Walter F.R. Weldon (1860–1906) founded the Biometrika scientific journal, which promoted biometrics and statistical analysis of heredity. Charles Davenport (1866–1944) was briefly involved in the review. In Race Crossing in Jamaica (1929), he made statistical arguments that biological and cultural degradation followed white and black interbreeding. Davenport was connected to Nazi Germany before and during World War II. In 1939 he wrote a contribution to the festschrift for Otto Reche (1879–1966), who became an important figure within the plan to remove populations considered "inferior" from eastern Germany. 
Scientific racism continued through the early 20th century, and soon intelligence testing became a new source for racial comparisons. Before World War II (1939–45), scientific racism remained common to anthropology, and was used as justification for eugenics programs, compulsory sterilization, anti-miscegenation laws, and immigration restrictions in Europe and the United States. The war crimes and crimes against humanity of Nazi Germany (1933–45) discredited scientific racism in academia, [ citation needed ] but racist legislation based upon it remained in some countries until the late 1960s.
Early intelligence testing and the Immigration Act of 1924
Before the 1920s, social scientists agreed that whites were superior to blacks, but they needed a way to prove this in order to back social policy in favor of whites. They felt the best way to gauge this was through testing intelligence. By interpreting the tests to show favor to whites these test makers' research results portrayed all minority groups very negatively.   In 1908, Henry Goddard translated the Binet intelligence test from French and in 1912 began to apply the test to incoming immigrants on Ellis Island.  Some claim that in a study of immigrants Goddard reached the conclusion that 87% of Russians, 83% of Jews, 80% of Hungarians, and 79% of Italians were feeble-minded and had a mental age less than 12.  Some have also claimed that this information was taken as "evidence" by lawmakers and thus it affected social policy for years.  Bernard Davis has pointed out that, in the first sentence of his paper, Goddard wrote that the subjects of the study were not typical members of their groups but were selected because of their suspected sub-normal intelligence. Davis has further noted that Goddard argued that the low IQs of the test subjects were more likely due to environmental rather than genetic factors, and that Goddard concluded that "we may be confident that their children will be of average intelligence and if rightly brought up will be good citizens".  In 1996 the American Psychological Association's Board of Scientific Affairs stated that IQ tests were not discriminatory towards any ethnic/racial groups. 
In his book The Mismeasure of Man, Stephen Jay Gould argued that intelligence testing results played a major role in the passage of the Immigration Act of 1924 that restricted immigration to the United States.  However, Mark Snyderman and Richard J. Herrnstein, after studying the Congressional Record and committee hearings related to the Immigration Act, concluded "the [intelligence] testing community did not generally view its findings as favoring restrictive immigration policies like those in the 1924 Act, and Congress took virtually no notice of intelligence testing". 
Juan N. Franco contested the findings of Snyderman and Herrnstein. Franco stated that even though Snyderman and Herrnstein reported that the data collected from the results of the intelligence tests were in no way used to pass The Immigration Act of 1924, the IQ test results were still taken into consideration by legislators. As suggestive evidence, Franco pointed to the following fact: Following the passage of the immigration act, information from the 1890 census was used to set quotas based on percentages of immigrants coming from different countries. Based on these data, the legislature restricted the entrance of immigrants from southern and eastern Europe into the United States and allowed more immigrants from northern and Western Europe into the country. The use of the 1900, 1910 or 1920 census data sets would have resulted in larger numbers of immigrants from southern and eastern Europe being allowed into the U.S. However, Franco pointed out that using the 1890 census data allowed congress to exclude southern and eastern Europeans (who performed worse on IQ tests of the time than did western and northern Europeans) from the U.S. Franco argued that the work Snyderman and Herrnstein conducted on this matter neither proved or disproved that intelligence testing influenced immigration laws. 
Following the creation of the first society for the promotion of racial hygiene, the German Society for Racial Hygiene in 1905—a Swedish society was founded in 1909 as "Svenska sällskapet för rashygien" as third in the world.   By lobbying Swedish parliamentarians and medical institutes the society managed to pass a decree creating a government run institute in the form of the Swedish State Institute for Racial Biology in 1921.  By 1922 the institute was built and opened in Uppsala.  It was the first such government-funded institute in the world performing research into "racial biology" and remains highly controversial to this day.   It was the most prominent institution for the study of "racial science" in Sweden.  The goal was to cure criminality, alcoholism and psychiatric problems through research in eugenics and racial hygiene.  As a result of the institutes work a law permitting compulsory sterilization of certain groups was enacted in Sweden in 1934.  The second president of the institute Gunnar Dahlberg was highly critical of the validity of the science performed at the institute and reshaped the institute toward a focus on genetics.  In 1958 it closed down and all remaining research was moved to the Department of medical genetics at Uppsala University. 
The Nazi Party and its sympathizers published many books on scientific racism, seizing on the eugenicist and antisemitic ideas with which they were widely associated, although these ideas had been in circulation since the 19th century. Books such as Rassenkunde des deutschen Volkes ("Racial Science of the German People") by Hans Günther  (first published in 1922)  and Rasse und Seele ("Race and Soul") by Ludwig Ferdinand Clauß [de]  (published under different titles between 1926 and 1934)  : 394 attempted to scientifically identify differences between the German, Nordic, or Aryan people and other, supposedly inferior, groups. [ citation needed ] German schools used these books as texts during the Nazi era.  In the early 1930s, the Nazis used racialized scientific rhetoric based on social Darwinism [ citation needed ] to push its restrictive and discriminatory social policies.
During World War II, Nazi racialist beliefs became anathema in the United States, and Boasians such as Ruth Benedict consolidated their institutional power. After the war, discovery of the Holocaust and Nazi abuses of scientific research (such as Josef Mengele's ethical violations and other war crimes revealed at the Nuremberg Trials) led most of the scientific community to repudiate scientific support for racism.
Propaganda for the Nazi eugenics program began with propaganda for eugenic sterilization. Articles in Neues Volk described the appearance of the mentally ill and the importance of preventing such births.  Photographs of mentally incapacitated children were juxtaposed with those of healthy children.  : 119 The film Das Erbe showed conflict in nature in order to legitimate the Law for the Prevention of Hereditarily Diseased Offspring by sterilization.
Although the child was "the most important treasure of the people", this did not apply to all children, even German ones, only to those with no hereditary weaknesses.  Nazi Germany's racially based social policies placed the improvement of the Aryan race through eugenics at the center of Nazis ideology. Those humans were targeted who were identified as "life unworthy of life" (German: Lebensunwertes Leben), including but not limited to Jewish people, criminals, degenerate, dissident, feeble-minded, homosexual, idle, insane, and the weak, for elimination from the chain of heredity. [ citation needed ] Despite their still being regarded as "Aryan", Nazi ideology deemed Slavs (i.e., Poles, Russians, Ukrainians, etc.) to be inferior to the Germanic master race, suitable for expulsion, enslavement, or even extermination.  : 180
Adolf Hitler banned intelligence quotient (IQ) testing for being "Jewish".  : 16
In the 20th century, concepts of scientific racism, which sought to prove the physical and mental inadequacy of groups deemed "inferior", was relied upon to justify involuntary sterilization programs.   Such programs, promoted by eugenicists such as Harry H. Laughlin, were upheld as constitutional by the U.S. Supreme Court in Buck v. Bell (1927). In all, between 60,000 and 90,000 Americans were subjected to involuntary sterilization. 
Scientific racism was also used as a justification for the Emergency Quota Act of 1921 and the Immigration Act of 1924 (Johnson–Reed Act), which imposed racial quotas limiting Italian American immigration to the United States and immigration from other southern European and eastern European nations. Proponents of these quotas, who sought to block "undesirable" immigrants, justifying restrictions by invoking scientific racism. 
Lothrop Stoddard published many racialist books on what he saw as the peril of immigration, his most famous being The Rising Tide of Color Against White World-Supremacy in 1920. In this book he presented a view of the world situation pertaining to race focusing concern on the coming population explosion among the "colored" peoples of the world and the way in which "white world-supremacy" was being lessened in the wake of World War I and the collapse of colonialism.
Stoddard's analysis divided world politics and situations into "white", "yellow", "black", "Amerindian", and "brown" peoples and their interactions. Stoddard argued race and heredity were the guiding factors of history and civilization, and that the elimination or absorption of the "white" race by "colored" races would result in the destruction of Western civilization. Like Madison Grant, Stoddard divided the white race into three main divisions: Nordic, Alpine, and Mediterranean. He considered all three to be of good stock, and far above the quality of the colored races, but argued that the Nordic was the greatest of the three and needed to be preserved by way of eugenics. Unlike Grant, Stoddard was less concerned with which varieties of European people were superior to others (Nordic theory), but was more concerned with what he called "bi-racialism", seeing the world as being composed of simply "colored" and "white" races. In the years after the Great Migration and World War I, Grant's racial theory would fall out of favor in the U.S. in favor of a model closer to Stoddard's. [ citation needed ]
An influential publication was The Races of Europe (1939) by Carleton S. Coon, president of the American Association of Physical Anthropologists from 1930 to 1961. Coon was a proponent of multiregional origin of modern humans. He divided Homo sapiens into five main races: Caucasoid, Mongoloid (including Native Americans), Australoid, Congoid, and Capoid.
Coon's school of thought was the object of increasing opposition in mainstream anthropology after World War II. Ashley Montagu was particularly vocal in denouncing Coon, especially in his Man's Most Dangerous Myth: The Fallacy of Race. By the 1960s, Coon's approach had been rendered obsolete in mainstream anthropology, but his system continued to appear in publications by his student John Lawrence Angel as late as in the 1970s.
In the late 19th century, the Plessy v. Ferguson (1896) United States Supreme Court decision—which upheld the constitutional legality of racial segregation under the doctrine of "separate but equal"—was intellectually rooted in the racism of the era, as was the popular support for the decision.  Later in the mid 20th century, the Supreme Court's Brown v. Board of Education of Topeka (1954) decision rejected racialist arguments about the "need" for racial segregation—especially in public schools.
By 1954, 58 years after the Plessy v. Ferguson upholding of racial segregation in the United States, American popular and scholarly opinions of scientific racism and its sociologic practice had evolved.  In 1960, the journal Mankind Quarterly started, which some have described as a venue for scientific racism. It has been criticized for a claimed ideological bias, and for lacking a legitimate scholarly purpose.  The journal was founded in 1960, partly in response to the Supreme Court decision Brown v. Board of Education which desegregated the American public school system.  
In April 1966, Alex Haley interviewed American Nazi Party founder George Lincoln Rockwell for Playboy. Rockwell justified his belief that blacks were inferior to whites by citing a long 1916 study by G. O. Ferguson which claimed to show that the intellectual performance of black students was correlated with their percentage of white ancestry, stating "pure negroes, negroes three-fourths pure, mulattoes and quadroons have, roughly, 60, 70, 80 and 90 percent, respectively, of white intellectual efficiency".  Playboy later published the interview with an editorial note claiming the study was a "discredited . pseudoscientific rationale for racism". 
International bodies such as UNESCO attempted to draft resolutions that would summarize the state of scientific knowledge about race and issued calls for the resolution of racial conflicts. In its 1950 "The Race Question", UNESCO did not reject the idea of a biological basis to racial categories,  but instead defined a race as: "A race, from the biological standpoint, may therefore be defined as one of the group of populations constituting the species Homo sapiens", which were broadly defined as the Caucasian, Mongoloid, Negroid races but stated that "It is now generally recognized that intelligence tests do not in themselves enable us to differentiate safely between what is due to innate capacity and what is the result of environmental influences, training and education." 
Despite scientific racism being largely dismissed by the scientific community after World War II, some researchers have continued to propose theories of racial superiority in the past few decades.   These authors themselves, while seeing their work as scientific, may dispute the term racism and may prefer terms such as "race realism" or "racialism".  In 2018, British science journalist and author Angela Saini expressed strong concern about the return of these ideas into the mainstream.  Saini followed up on this idea with her 2019 book Superior: The Return of Race Science. 
One such post-World War II scientific racism researcher is Arthur Jensen. His most prominent work is The g Factor: The Science of Mental Ability in which he supports the theory that black people are inherently less intelligent than whites. Jensen argues for differentiation in education based on race, stating that educators must "take full account of all the facts of [students'] nature."  Responses to Jensen criticized his lack of emphasis on environmental factors.  Psychologist Sandra Scarr describes Jensen's work as "conjur[ing] up images of blacks doomed to failure by their own inadequacies". 
J. Philippe Rushton, president of the Pioneer Fund (Race, Evolution, and Behavior) and a defender of Jensen's The g Factor,  also has multiple publications perpetuating scientific racism. Rushton argues "race differences in brain size likely underlie their multifarious life history outcomes."  Rushton's theories are defended by other scientific racists such as Glayde Whitney. Whitney published works suggesting higher crime rates among people of African descent can be partially attributed to genetics.  Whitney draws this conclusion from data showing higher crime rates among people of African descent across different regions. Other researchers point out that proponents of a genetic crime-race link are ignoring confounding social and economic variables, drawing conclusions from correlations. 
Christopher Brand was a proponent of Arthur Jensen's work on racial intelligence differences.  Brand's The g Factor: General Intelligence and Its Implications claims black people are intellectually inferior to whites.  He argues the best way to combat IQ disparities is to encourage low-IQ women to reproduce with high-IQ men.  He faced intense public backlash, with his work being described as a promotion of eugenics.  Brand's book was withdrawn by the publisher and he was dismissed from his position at the University of Edinburgh.
Psychologist Richard Lynn has published multiple papers and a book supporting theories of scientific racism. In IQ and the Wealth of Nations, Lynn claims that national GDP is determined largely by national average IQ.  He draws this conclusion from the correlation between average IQ and GDP and argues low intelligence in African nations is the cause of their low levels of growth. Lynn's theory has been criticized for attributing causal relationship between correlated statistics.   Lynn supports scientific racism more directly in his 2002 paper "Skin Color and Intelligence in African Americans", where he proposes "the level of intelligence in African Americans is significantly determined by the proportion of Caucasian genes."  As with IQ and the Wealth of Nations, Lynn's methodology is flawed, and he purports a causal relationship from what is simply correlation. 
Other prominent modern proponents of scientific racism include Charles Murray and Richard Herrnstein (The Bell Curve) and Nicholas Wade (A Troublesome Inheritance). Wade's book faced strong backlash from the scientific community, with 142 geneticists and biologists signing a letter describing Wade's work as "misappropriation of research from our field to support arguments about differences among human societies." 
On 17 June 2020, Elsevier announced it was retracting an article that J. Philippe Rushton and Donald Templer had published in 2012 in the Elsevier journal Personality and Individual Differences.  The article falsely claimed that there was scientific evidence that skin color was related to aggression and sexuality in humans. 
What is a species?
The most famous definition of a species comes from the 20th century German-born biologist Ernst Mayr, who emphasised the importance of interbreeding. The idea (roughly) is that two organisms are of the same species if they can breed with one another to produce fertile offspring. That is why a donkey and a horse aren’t the same species: they can breed and produce offspring, but not fertile offspring.
Mayr’s way of thinking about species has some amazing consequences. Recently, due to rising temperatures in the Arctic, polar bears and grizzly bears have been coming into increased contact, and have been producing fertile offspring. The offspring are (adorably) called grolar or pizzly bears. What this suggests is that polars and grizzlies may actually be the same species after all, despite radical differences in size, appearance, hibernation behaviours, diet and so on.
But it wasn’t long before the problems with Mayr’s approach became apparent. The definition makes use of the notion of interbreeding. This is all very well with horses and polar bears, but smaller organisms like bacteria do not interbreed at all. They reproduce entirely asexually, by simply splitting in two. So this definition of species can’t really apply to bacteria. Perhaps when we started thinking about species in terms of interbreeding, we were all just a bit too obsessed with sex.
Ernst Haeckel’s (1866) conception of the three kingdoms of life. Wikimedia Commons
So maybe we should forget about sex and look for a different approach to species. In the 1960s, another German biologist, Willi Hennig, suggested thinking about species in terms of their ancestry. In simple terms, he suggested that we should find an organism, and then group it together with its children, and its children’s children, and its children’s children’s children. Eventually, you will have the original organism (the ancestor) and all of its descendents. These groups are called clades. Hennig’s insight was to suggest that this is how we should be thinking about species.
But this approach faces its own problems. How far back should you go before you pick the ancestor in question? If you go back in history far enough, you’ll find that pretty much every animal on the planet shares an ancestor. But surely we don’t want to say that every single animal in the world, from the humble sea slug, to top-of-the-range apes like human beings, are all one big single species?
A new population that results from a speciation event is called a species. But although species result from a simple process, recognizing species in nature can be complicated. Biologists cannot travel in time to observe the speciations that resulted in today's diversity of life, so they must observe the reproduction of living organisms to determine the makeup of species. Paleontologists can find the fossil evidence of the ancestors of today's species, but they cannot observe whether those fossil organisms could reproduce with each other. Because scientists have different kinds of evidence about organisms, they use different concepts of species when testing hypotheses about their evolution.
The most obvious property that helps to define species is reproductive isolation. Biologists studying living animals often use the biological species concept, which envisions a species as a "group of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups" (Mayr 1942). It is the biological species concept that primatologists use to grapple with whether chimpanzees and bonobos are different species, for example, by observing the differences in their reproductive behaviors and the strength of geographic isolation between their populations.
The biological species concept has some important limitations for paleontology. Making use of the concept depends on observing the mating behavior and interbreeding patterns of animals in their natural environments, which is not possible with fossils of organisms that lived in the past. Other kinds of observations that paleontologists might gather, such as morphological differences between fossils, have no necessary value under this concept. Another limitation is that the biological species concept does not incorporate any idea of how species may change over time. Paleontologists study fossils that may be separated by hundreds of thousands of years of time. It is difficult to imagine such widely separated individuals as part of the same reproductive community, even if they were very similar to each other. Over such time periods, evolution can transform populations substantially. The biological species concept recognizes the genetic continuity within a species caused by gene flow, but it does not incorporate a view of species existing over evolutionary time. For these reasons, paleontology requires a different kind of species concept.
Phylogenetic species concept
The phylogenetic species concept is an attempt to define species by their relationships to other species. Instead of trying to determine the reproductive boundaries of populations, scientists using the phylogenetic species concept attempt to uncover their genealogical relationships. A group of individuals that includes all the descendants of one common ancestor, leaving no descendants out, is called a monophyletic group.
Paleontologists Niles Eldredge and Joel Cracraft devised a species concept called the "Phylogenetic Species Concept," intended to apply to circumstances in which reproduction or isolation among organisms could not be observed. Under this concept, a species is "a diagnosable cluster of individuals within which there is a parental pattern of ancestry and escent, beyond which there is not, and which exhibits a pattern of phylogenetic ancestry and descent among units of like kind" (Eldredge and Cracraft 1980:92).
Key to the phylogenetic species concept is the idea that species must be "diagnosable." In other words, members of the species should share a combination of characteristics that other species lack. To look for the unique features that define a phylogenetic species, paleontologists must perform systematic comparisons with other related fossils or living species. These aspects of the concept make it widely applicable in paleontology.
But the phylogenetic species concept is not without its problems. Because the concept defines species based on morphology, without explicitly referring to populations or reproductive boundaries, it does not apply well to cases where morphologically different populations are connected by gene flow. Morphological variation among populations is not uncommon within living species. Humans today are a species with substantial morphological variation from continent to continent. Humans on different continents are not reproductively isolated, and their variation is largely distributed as clines over large geographic distances. Yet a paleontologist who had only a few fragmentary specimens from each continent would not necessarily know the pattern of variation, and many features of his specimens would appear to be unique. What would the paleontologist make of the high nose of a European specimen, the forward-facing cheeks of an Asian fossil, or the strong browridge above the eye orbits of an Australian, each taken randomly from their variable populations? By applying the phylogenetic species concept, a paleontologist would probably conclude that the different continents were homes to different human species.
Thus, because the phylogenetic species concept does not identify species based on the reproductive boundaries between them, it may have the effect of identifying populations connected by gene flow as different species. For this reason, a phylogenetic species as defined by a paleontologist may not correspond to a real prehistoric population that was the product of a speciation. Some paleontologists do not view this potential conflict as a problem, because identifying species based on unique characteristics will create as full as possible a systematization of the evolution of new features. Assuming that the number of ancient species was very large, and the number of fossils representing each of them is very small, then paleontologists can hardly hope to identify every speciation event in the past. The phylogenetic species concept may therefore provide a better approximation of the number and diversity of species that existed than other alternatives.
On the other hand, identifying populations connected by gene flow as different species can be a significant problem for paleontologists who take a greater interest in the processes of evolution than in the diversity of species in the past. Gene flow is a significant force shaping evolutionary change within populations. Moreover, evolution may cause a single species to change over time, possibly acquiring new unique features without any division of a species into separate reproductively isolated populations. Some paleontologists approach these difficulties by altering their view of the evolutionary process. If speciations can happen as a transformation of a single population in addition to the appearance of reproductive boundaries between populations, then a single evolving population may over time comprise several phylogenetic species. Or if most evolutionary change happened at the time of speciation, as asserted by the concept of punctuated equilibrium, then the phylogenetic species concept might more closely approximate the actual pattern of speciations in the past. But without such assumptions, the phylogenetic species concept's problems sometimes create a stumbling block for some paleontologists in attempting to understand the evolutionary process.
The evolutionary species concept combines the genealogical basis of the phylogenetic species concept with the genetic basis of the biological species concept. An evolutionary species is a lineage of interbreeding organisms, reproductively isolated from other lineages, that has a beginning, an end, and a distinct evolutionary trajectory (Wiley 1978). The beginning of a species' existence is a speciation, as a population becomes reproductively isolated from a parent population. The end of a species occurs either with extinction or with the branching of the species into one or more descendants.
Central to the evolutionary species concept is the idea of an evolutionary trajectory. The trajectory of a species is the evolutionary pattern of its characteristics over time. For example, one of the earliest species in the story of human evolution, Australopithecus afarensis, is represented by dozens of fossil teeth and mandibles, as well as other remains. Paleontologists hypothesize that these fossils, from several sites in East Africa, are members of a single species because of their many morphological resemblances. No very similar fossils have ever been found before 3.6 million or after 3 million years ago, dates that appear to indicate the beginning and the end of the species.
Nevertheless, the fossils do show some differences that appear over time. Although the molar teeth of the fossils do not change over time, the mandibles are thicker and more massive in more recent fossils than in the most ancient ones. As far as paleontologists can test, the mandibles form a single series evolving over time toward greater size and thickness. The evolutionary species concept infers that the fossils represent a species, beginning 3.6 million years ago and ending 3 million years ago, with an evolutionary trajectory that includes the evolution of greater mandibular thickness, without apparent changes in molar sizes.
The strength of the evolutionary species concept is that it allows paleontologists to focus on the causes of evolutionary change, whether they occur during speciations or at other times. Regarding A. afarensis, the observation that mandibles increased in size during the existence of the species may be explained by different evolutionary forces and conditions than if all the change occurred with the reproductive isolation of a new population. Although the greater mandibular thickness of later mandibles might be a unique feature, attempting to establish a new phylogenetic species for the later fossils might detract from an explanation of the overall evolutionary pattern.
Phylogenetic species vs. evolutionary species concepts
Yet for many paleontologists, the need to amass great numbers of fossils from different times makes the evolutionary species concept nearly impossible to implement. At the same time, if scientists always hold out the possibility that two different fossils were actually connected by gene flow, it may impede an understanding of evolutionary changes that accompany the appearance of new reproductively isolated species. If we want to have a scientific, meaning falsificationist, view of the species that have existed and their boundaries and relationships to each other, we must accept that the process will in many cases be difficult. Simply making up many species hypotheses cannot add to our knowledgeand in many cases it may detract. What is important is that we realize that our record of past species is incomplete, and our failure to substantiate the existence of many species in the past does not constitute evidence that they did not exist.
Testing species hypotheses
However species are defined, whenever scientists identify a species, they actually are stating a hypothesis about the relationships among individual organisms. Such a hypothesis may be tested using morphological, genetic, or behavioral evidence. Discovering real species that existed in the past involves predicting the morphological variability of populations, including variation that occurs among populations connected by gene flow. In the relatively small fossil samples available to paleontologists, determining the number of species in a sample is a significant problem. Researchers use a number of techniques to test species hypotheses with limited morphological samples.
Two fossil hominids: different species or not?
- What is the level of morphological difference between two or more specimens? Using a living species for comparison, scientists can determine the likelihood of sampling similar variability as the fossil sample (Miller 2000).
- What are the relative frequencies of characteristics in two samples of fossils? Statistical comparison with the differences between different populations within a living species can determine whether the differences in frequencies observed in the fossils would be likely to occur within the comparison species. Such comparisons can be extended to the differences between the sexes of a living species to test whether sexual dimorphism accounts for differences between fossils (Lee 1999).
- How do morphological features covary? If one fossil sample has a high incidence of several features that are absent or at low frequency in another sample, this supports the hypothesis that the two samples represent different species. With samples of sufficient size, say, 10 individuals or more, paleontologists can even estimate the maximum level of gene flow consistent with the morphological differences, and thereby frame a test of the hypothesis of different species in solid evolutionary terms (Hawks and Wolpoff 2001).
- Do samples represent change over time? Sometimes paleontologists can use different populations from living species to evaluate likelihood that certain kinds of changes might occur over time. The best comparisons are with large samples of fossils that represent long spans of time, however. Although the evolutionary process is in ways unique for each species, analyses of the rate and level of changes in other species provide the most powerful tests of species hypotheses available in studying the past.
Eldredge N, Cracraft J. 1980. Phylogenetic patterns and the evolutionary process: Method and theory in comparative biology. New York: Columbia University Press.
Hawks J, Wolpoff MH. 2001. The accretion model of Neandertal evolution. Evolution 55:1474-1485. PubMed
Lee SH. 1999. Evolution of human sexual dimorphism: Using assigned resampling method to estimate sexual dimorphism when individual sex is unknown. Ph.D. thesis, University of Michigan.
Mayr E. 1942. Systematics and the origin of species from the viewpoint of a zoologist. New York: Columbia University Press.
Miller JMA. 2000. Craniofacial variation in Homo habilis: An analysis of the evidence for multiple species. Am J Phys Anthropol 112:103-128. PubMed
Wiley EO. 1978. The evolutionary species concept reconsidered. Syst Zool 27:17-26.
Updated: February 8, 2005
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18.2A: The Biological Species Concept - Biology
The biological species concept has its limitations (although it works well for many organisms and has been very influential in the growth of evolutionary theory). In order to address some of these limitations, many other "species concepts" have been proposed, such as:
- Recognition species concept: a species is a set of organisms that can recognize each other as potential mates.
Even though these two frogs have been prevented from mating, the fact that they recognize each other as potential mates makes them the same species under the recognition species concept.
According to this concept, phenotypic similarity is all that matters in recognizing separate species. Since the frogs depicted here look the same even though they are prevented from mating with each other they would be considered the same species according to the phenetic species concept.
In this example, Ensatina salamander lineages A and B are separate species. Each has a common ancestor that individuals of other species do not. Even though it has diversified a lot, Lineage C is a single species, according to the phylogenetic species concept. None of the subspecies of Lineage C has a single common ancestor separate from the other subspecies.
A researcher's choice of species concept often reflects his or her research focus. Making that decision requires the scientist to commit to a species concept. For most purposes and for communication with the general public, the biological species concept is used.
Enzymes and Nucleic Acids
B. Catalog of mutations
A mutation may be associated with a change in a biological trait (s) called phenotype. [The genetic trait giving rise to the phenotype is called genotype. Genotypes are described using italic type, e.g., lac, whereas the phenotypes, which are described by the same terms, are not italicized but the first letter is capitalized, e.g., Lac.] The usual active form of a gene or a phenotypic feature is referred to as wild-type. A mutation that results in no phenotypic changes is called a silent mutation.
When a codon specific for one amino acid is changed to a codon specific for another by single-base replacements or point mutations, it is called a missense mutation. When a mutation involves substitution of one Py by another Py, it is called a transition. The substitution between Pu and Py is called a transversion. Transversion mutation is less frequent than transition mutation.
Missense mutant proteins/enzymes may have full, partially impaired, or null (biological) activities depending on the location of the mutation. When the functional impairment manifests itself only at a higher or lower than “normal” temperature that is considered optimal for the wild-type enzyme, the mutation is called temperature sensitive (ts). Although most ts mutants are sensitive to higher than normal temperatures, some ts mutants are cold sensitive. When an amino acid codon is changed to a termination codon, it is called a nonsense mutation. Nonsense mutations result in incomplete or truncated polypeptides of varying sizes and functional profiles.
With respect to cell viability, a mutation can be lethal, conditionally lethal, or nonlethal. Conditionally lethal mutations (e.g., ts mutations) are lethal under one set of conditions (nonpermissive) whereas they exhibit no critical deficiency for cell viability under alternative or permissive conditions.
Mutations also occur from gene rearrangements and errors in gene recombinations that result in either insertions or deletions of one or more nucleotides. Insertions or deletions may lead to a change in the codon reading frame. From a biological perspective, insertions and deletions are part of the normal cellular processes that have evolved as a control mechanism of gene expression (refer to Section II,D,2 , in this chapter). A mutation arising from the lack of an entire gene is termed a null mutation.
Viruses often present a special class of mutants called escape mutants. The mutation(s) occurs at neutralizing immunogenic sites such that the alteration of antigenicity allows the virus to escape from the neutralizing antibodies of the host. Influenza viruses are notorious for their propensity to undergo such mutations, which are specifically called antigenic drift.
Biological species concept
biological species concept (BSC) The concept of a species as a group of populations whose members are capable of interbreeding successfully and are reproductively isolated from other groups. This concept became influential during the late 19th and early 20th centuries, largely replacing the typological species concept favoured by pioneer naturalists. Central to the concept is the role of sexual reproduction. This maintains the broad uniformity of species' members through genetic recombination and sharing of a common gene pool. Isolating mechanisms prevent breeding, and hence gene flow, between different groups, thus ensuring genetic divergence between groups. However, the concept cannot be applied to exclusively asexual organisms, such as certain groups of fungi and bacteria. Nor does it account satisfactorily for the many instances in which interspecies mating does occur, especially in plants, fungi, and prokaryotes.
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D OES I T M ATTER? I MPLICATIONS OF O UR ESC C OMPARED TO A P URE L INEAGE A PPROACH
Beyond the theoretical issues of how we may view species, the question arises as to what extent the differences matter. We argue that they do matter in the sense that we perceive more or less diversity in a place or clade, differently assess imperiled populations, and make conclusions about the evolutionary process differently. In this section, we detail some of the differences with respect to our ESC approach compared to a pure lineage approach.
One way in which the difference matters is in implications for neontological and paleontological views of species, which have often been seen as being in tension ( Thomas 1956 Imbrie 1957 Simpson 1961). In the extreme, Simpson (1951), as noted above, argued that paleontological and neontological species are fundamentally different. How this can be so is unclear to us, since presumably the same sort of species has existed at any particular time. Including role as an inherent part of the species concept has the benefit of reconciling these perspectives, since role differs among species at a particular time and changes with time because it is directly dependent on character change. Empirically, phenotype can be evaluated both in and through time, meaning that the same criteria apply to species in both the time-limited and time-extended views.
Another practical difference lies in the treatment of geographically isolated populations. Under a concept that strictly equates species with lineages, persistently geographically isolated populations that are otherwise no different from other populations are distinct species, since it is the absence of actual interbreeding among populations that generates and defines lineages. Hence, the notion of a “disjunct population” of a species ceases to exist under a pure lineage concept. The HSC and GLC would fall into this camp. Although the BSC is otherwise similar to a pure lineage concept in relying on actual interbreeding in many cases, presumably neither Mayr (1942) nor Dobzhansky (1935) would have agreed with elimination of the idea that the ability to interbreed (or not) was important in their definitions, emphasizing as they did the importance of intrinsic isolating mechanisms. Acceptance of persistently geographically isolated populations with no other differences as species has been viewed as problematic by some authors ( Van Valen 1976 Mallet 1995 Wiens 2004) and would certainly lead to a proliferation of species relative to those that are commonly circumscribed today.
Situations in which species circumscription can be challenging. Branch length indicates degree of historical change (lineage formation), whereas lateral distance among clades indicates degree of phenotypic differentiation. a) Lineage formation with little phenotypic change. b) Phenotypic change with little lineage formation. c) The progenitor–derivative scenario, here with two lineages emerging from a progenitor. Dashed ovals indicate the species we would circumscribe in each case.
Situations in which species circumscription can be challenging. Branch length indicates degree of historical change (lineage formation), whereas lateral distance among clades indicates degree of phenotypic differentiation. a) Lineage formation with little phenotypic change. b) Phenotypic change with little lineage formation. c) The progenitor–derivative scenario, here with two lineages emerging from a progenitor. Dashed ovals indicate the species we would circumscribe in each case.
1. Lineage formation with little or no phenotypic change ( Fig. 2a): Sometimes called “non-adaptive radiation” ( Gittenberger 1991), this pattern results in historical patterning of lineages through time, but apparently with little biotic or abiotic selective pressure to fix changes. This is the case that is often being termed “cryptic species,” where detected gene lineages are perceived as significant, even with no phenotypic differences. Such lineages within species have been the domain of phylogeography ( Avise 2000) but are now being recognized as more significant because of a greater ability to resolve structure with larger amounts of data and because of the emphasis on gene tree monophyly. While these lineages are interesting phylogeographically, they are of little significance as taxa, and unless phenotypic differences can be found we argue that they should not be recognized as species.
2. Phenotypic change with little historical signature ( Fig. 2b): This involves the decoupling of role or phenotypic distinction from lineage formation as it is viewed from a gene tree perspective, and is likely the result of a few loci that have cascading phenotypic effects or are under strong selection. A number of recent studies have shown the surprising degree to which phenotypic divergence can occur even in the presence of substantial gene flow ( Morjan and Rieseberg 2004 Rieseberg et al. 2004 Jordan et al. 2005 Poelstra et al. 2014), meaning that population boundaries may be more porous than we expect even when phenotypic distinction exists. In these cases, we might expect lineage estimation from gene trees to indicate a lack of clear pattern even though distinct phenotypes persist as fixed characters in population groups. This may also be true in adaptive radiations, where distinct morphological types may be present with little evidence of genetic lineage formation ( Barley et al. 2013). Unless there is evidence that a particular phenotypic type is arising independently in different populations, we would recognize these types as species.
3. Progenitor–derivative species ( Fig. 2c): The case of a progenitor–derivative relationship, in which a subset of a former metapopulation lineage becomes distinct for a novel phenotype and forms a new lineage while leaving the ancestral type with the plesiomorphic features, is problematic for the coalescence approach. The problem occurs when the two resulting metapopulations are highly dissimilar in size, resulting in very different coalescence times. Paraphyletic assemblages of populations are in fact what one would expect in such scenarios, specifically with peripatric speciation or speciation following long-distance dispersal the expectation of these patterns has been described by previous authors (e.g., Rieseberg and Brouillet 1994 Crisp and Chandler 1996 Harrison 1998 Hudson and Coyne 2002). Though not monophyletic for its gene trees, the “paraphyletic residuum” may easily be a lineage sensu de Queiroz. With enough time, the assumption is that the patterns will resolve to “reciprocal monophyly” due to chance extinction of individuals, but this can be a very long time in groups with large effective population size ( |$N_ Such paraphyletic species are not just hypothetical constructs. Molecular systematic and evolutionary studies recover patterns of paraphyletic species with some frequency and some authors are concerned about the perceived “problem” ( Harrison 1998). Studies by, for example, Patton and Smith (1994), Nikulina et al. (2007), Syring et al. (2007), Feinstein et al. (2008), Martinsen et al. (2009), and Kadereit et al. (2012) have revealed such patterns. Some authors have even expressed concern about conflict with nomenclatural codes, suggesting that “Paraphyly is difficult to reconcile with a nomenclatural Code that presumes that all individuals are members of monophyletic groups” ( Grube and Kroken 2000), although monophyly is not even mentioned in the current botanical or zoological codes ( McNeill et al. 2012 ICZN 1999). While the molecular pattern should prompt researchers to investigate these groups more carefully, especially seeking phenotypic features that may correlate with lineage groups, under our view there is no reason to reject species status for paraphyletic assemblages of populations we argue that they should not be denied species status in the intervening period while waiting for genetic coalescence to occur. Ultimately, the way that we view species influences how we view evolution. In our way of viewing species, speciation is the fixation of a new role in a lineage, whereas under pure lineage concepts speciation is population division ( |$=$| cessation of interbreeding) the possibility of anagenesis does not exist there. We do not argue that true anagenetic speciation is necessarily common or easy to demonstrate, but its possibility should not be ruled out and our approach accommodates it. Although our focus here has been on sexually reproducing organisms, much of biodiversity comprises asexual individuals (notably bacteria and archaea). Workers dealing with those organisms have struggled to define a meaningful species concept, but recent arguments have been made for something similar to what we advocate here—combining elements of genetic relatedness and phenotype (e.g., Rosselló-Móra and Amann 2001, 2015). Asexual organisms form clear lineages and to the extent that they differ in role, our approach to species should be applicable to them as well as to sexual groups. It was not our intention to propose a new species concept here as with de Queiroz, we believe that all of the elements already exist in previous proposals. We see the way of viewing species outlined here as only a modification of some previous well-known ones. We view it as a clarification of Simpson’s ESC and Van Valen’s Ecological Species Concept. Sandler (2012), in his treatment of the ethics of species, used as a working concept, “groups of biologically related organisms that are distinguished from other groups of organisms by virtue of their shared form of life. A species’ form of life refers to how individuals of the biological group typically strive to make their way in the world.” This is in essence our view, since it includes both relationship and role. If it needs a name to distinguish it, we would suggest the “phenophyletic” view, emphasizing the dual aspects of role/phenotype and lineage that it embodies. On balance, we argue that this concept of species aligns better with ideas of biodiversity, reconciles notions of species in time, and yields an entity of more general significance than that resolved under a strict lineage concept.
Such paraphyletic species are not just hypothetical constructs. Molecular systematic and evolutionary studies recover patterns of paraphyletic species with some frequency and some authors are concerned about the perceived “problem” ( Harrison 1998). Studies by, for example, Patton and Smith (1994), Nikulina et al. (2007), Syring et al. (2007), Feinstein et al. (2008), Martinsen et al. (2009), and Kadereit et al. (2012) have revealed such patterns. Some authors have even expressed concern about conflict with nomenclatural codes, suggesting that “Paraphyly is difficult to reconcile with a nomenclatural Code that presumes that all individuals are members of monophyletic groups” ( Grube and Kroken 2000), although monophyly is not even mentioned in the current botanical or zoological codes ( McNeill et al. 2012 ICZN 1999). While the molecular pattern should prompt researchers to investigate these groups more carefully, especially seeking phenotypic features that may correlate with lineage groups, under our view there is no reason to reject species status for paraphyletic assemblages of populations we argue that they should not be denied species status in the intervening period while waiting for genetic coalescence to occur.
Ultimately, the way that we view species influences how we view evolution. In our way of viewing species, speciation is the fixation of a new role in a lineage, whereas under pure lineage concepts speciation is population division ( |$=$| cessation of interbreeding) the possibility of anagenesis does not exist there. We do not argue that true anagenetic speciation is necessarily common or easy to demonstrate, but its possibility should not be ruled out and our approach accommodates it.
Although our focus here has been on sexually reproducing organisms, much of biodiversity comprises asexual individuals (notably bacteria and archaea). Workers dealing with those organisms have struggled to define a meaningful species concept, but recent arguments have been made for something similar to what we advocate here—combining elements of genetic relatedness and phenotype (e.g., Rosselló-Móra and Amann 2001, 2015). Asexual organisms form clear lineages and to the extent that they differ in role, our approach to species should be applicable to them as well as to sexual groups.
It was not our intention to propose a new species concept here as with de Queiroz, we believe that all of the elements already exist in previous proposals. We see the way of viewing species outlined here as only a modification of some previous well-known ones. We view it as a clarification of Simpson’s ESC and Van Valen’s Ecological Species Concept. Sandler (2012), in his treatment of the ethics of species, used as a working concept, “groups of biologically related organisms that are distinguished from other groups of organisms by virtue of their shared form of life. A species’ form of life refers to how individuals of the biological group typically strive to make their way in the world.” This is in essence our view, since it includes both relationship and role. If it needs a name to distinguish it, we would suggest the “phenophyletic” view, emphasizing the dual aspects of role/phenotype and lineage that it embodies. On balance, we argue that this concept of species aligns better with ideas of biodiversity, reconciles notions of species in time, and yields an entity of more general significance than that resolved under a strict lineage concept.