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Smallest unit on which selection can act

Smallest unit on which selection can act



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Traditionally, the individual was considered to be the smallest unit on which Natural Selection (NS) acts. Today, we usually consider the gene as being the unit of NS. Of course, we should also consider all sequences that affect the fitness even though they are not genes (even though the do not code for polypeptide). And theoretically, any sequence of DNA does have an effect on fitness because it influences the time and energy for DNA replication (although it might be negligible). The decision of considering the gene as the smallest unit of NS seems rather arbitrary to me. We might as well consider a group of genes or a given exon of even a smaller sequence.

Here are my questions:

  • What factors influence the minimal size of a sequence to be considered as a unit on which NS acts? Mutation rate, generation time, selection differential for this sequence, recombination rate,… ?

  • Could we consider a nucleotide as a unit of NS? Why?

  • How does the quasispecies model fit into the question of what is the smallest unit of NS? (for those interested, you will also find a very good explanation of this model in Martin Nowak's book called Evolutionnary Dynamic: exploring the equations of life)

  • Is it worth talking about that? Is this question biologically relevant? Or is it rather a question based on a choice of definition such as "Is a virus alive?"

As I asked several questions, let me know if I should split my post into several. Otherwise, please do not hesitate to answer only very partially to this post!

UPDATE

Terdon's answer makes sense to me. I should be a bit more accurate in the reas of my question. I read The extended Phenotype from Richard Dawkins quite a long time ago and if I'm not mistaken, Dawkins says the following things

A unit on which selection acts has to be:

  • active
  • germ-line
  • replicator

A replicator has the 3 following properties:

  • fecundity
  • longevity
  • fidelity while being copied

Therefore for fidelity to be respected a unit of selection has to be a sequence which is not too long, so that it is not too often modified by recombination or mutation. He argues in this sense.

He also argues that nucleotides are not possible unit of selection. Indeed, it is hard to imagine a nucleotide being an active replicator. The word active means that it influences its not probability to be replicated. I don't think a nucleotide can do such a thing.

Unfortunately I don't have the book with me right now and I can't check what I have said, give you a citation nor a more accurate a reference. If anyone has some citations from this book, it will be welcome for the discussion!

Thank you!


The smallest unit that can be selected is, of course, the single nucleotide. The most striking examples of this are Single Nucleotide Polymorphisms (SNPs), many of which confer selective (dis)advantages.

To take a simple example, imagine a SNP that introduces a frameshift mutation, rendering a gene incapable of producing its protein. If that protein is something relatively important like p53, the SNP in question will be lethal and will be selected against. So, to take your questions one by one:

  1. What factors influence the minimal size of a sequence to be considered as a unit on which NS acts? Mutation rate, generation time, selection differential for this sequence, recombination rate,… ?

    • any sequence unit whose alteration can affect fitness is a candidate for NS.
  2. Could we consider a nucleotide as a unit of NS? Why?

    • Yes, if it can affect fitness (which it can, see above) it can be selected for or against.
  3. How does the quasispecies model fit into the question of what is the smallest unit of NS?

    • As far as I can tell, it is irrelevant. NS can act on any self-replicating entity as long as the process of replication can produce variation that can affect fitness. If I understand correctly, the quasispecies model simply posits a group of entities with a very very high mutational rate. However, the rate of mutation does not affect the smallest unit that NS can act on, as I said above, anything that can affect fitness will be subject to NS.
  4. Is it worth talking about that? Is this question biologically relevant? Or is it rather a question based on a choice of definition such as "Is a virus alive?"

    Yes, it is worth talking about it if you have the misconception that the smallest unit is a gene :). No, but seriously, it is an interesting concept and a very useful tool. The selfish gene hypothesis for example, helped us see the world of evolution in terms of sequences rather than species or individuals. NS did not change, our conception of it did.

These are all useful conceptual tools that allow us as scientists to understand complex questions. However, NS is not a directed or conscious process so it does not care or know what it acts on.


PS. Please do not confuse protein-coding sequences with genes. As I said in my comment, there are many genes that do not produce proteins (tRNA genes for example) and that was even before the waters were muddied in the past few years. The current definition of gene is something like:

any nucleotide sequence with a job to do

Read the ENCODE papers for a more technical definition or take this one from wikipedia:

A gene is a molecular unit of heredity of a living organism.


UPDATE: First of all, you have to remember that Dawkins wrote The Extended Phenotype more than 30 years ago, in a time when the only complete genomes available were those of a few viruses. The first bacterial genome was sequenced in the mid nineties and the first human genome draft human in 2000. It was a time when selection was seen as acting at the level of the individual, and the importance of genes was underestimated. Dawkins books were instrumental in bringing genes into the spotlight.

However, if he did argue that a nucleotide cannot be selected (I have not read this particular book) he was simply wrong. A nucleotide can indeed affect its chances of being replicated. Think about it, if you have aGat a given position in the genome, and thatGis mutated to anA, then the chances of the originalGbeing replicated are obviously very low. Therefore, the nucleotide 'affected' its chances of replication. Alternatively, a mutation of a single nucleotide in the promoter of a gene can also cause the entire gene not to be transcribed and, as I mentioned before, frameshift mutations can wreak havoc as well. All of these are down to single nucleotides.

In any case, our vision of what a gene is has changed enormously since Dawkins wrote his book, we understand the genome much better now, and we have realized that it is far more complex than originally believed. 30-year old literature about genes, even if written by such luminaries as Rihard Dawkins, should be taken with a pinch of salt today.


Natural selection

Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the heritable traits characteristic of a population over generations. Charles Darwin popularised the term "natural selection", contrasting it with artificial selection, which in his view is intentional, whereas natural selection is not.

Variation exists within all populations of organisms. This occurs partly because random mutations arise in the genome of an individual organism, and their offspring can inherit such mutations. Throughout the lives of the individuals, their genomes interact with their environments to cause variations in traits. The environment of a genome includes the molecular biology in the cell, other cells, other individuals, populations, species, as well as the abiotic environment. Because individuals with certain variants of the trait tend to survive and reproduce more than individuals with other less successful variants, the population evolves. Other factors affecting reproductive success include sexual selection (now often included in natural selection) and fecundity selection.

Natural selection acts on the phenotype, the characteristics of the organism which actually interact with the environment, but the genetic (heritable) basis of any phenotype that gives that phenotype a reproductive advantage may become more common in a population. Over time, this process can result in populations that specialise for particular ecological niches (microevolution) and may eventually result in speciation (the emergence of new species, macroevolution). In other words, natural selection is a key process in the evolution of a population.

Natural selection is a cornerstone of modern biology. The concept, published by Darwin and Alfred Russel Wallace in a joint presentation of papers in 1858, was elaborated in Darwin's influential 1859 book On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life. He described natural selection as analogous to artificial selection, a process by which animals and plants with traits considered desirable by human breeders are systematically favoured for reproduction. The concept of natural selection originally developed in the absence of a valid theory of heredity at the time of Darwin's writing, science had yet to develop modern theories of genetics. The union of traditional Darwinian evolution with subsequent discoveries in classical genetics formed the modern synthesis of the mid-20th century. The addition of molecular genetics has led to evolutionary developmental biology, which explains evolution at the molecular level. While genotypes can slowly change by random genetic drift, natural selection remains the primary explanation for adaptive evolution.


Contents

Charles Darwin developed the theory of evolution in his book, Origin of Species. Darwin also made the first suggestion of group selection in The Descent of Man that the evolution of groups could affect the survival of individuals. He wrote, "If one man in a tribe. invented a new snare or weapon, the tribe would increase in number, spread, and supplant other tribes. In a tribe thus rendered more numerous there would always be a rather better chance of the birth of other superior and inventive members." [3] [4]

Once Darwinism had been accepted in the modern synthesis of the mid-twentieth century, animal behavior was glibly explained with unsubstantiated hypotheses about survival value, which was largely taken for granted. The naturalist Konrad Lorenz had argued loosely in books like On Aggression (1966) that animal behavior patterns were "for the good of the species", [1] [5] without actually studying survival value in the field. [5] Richard Dawkins noted that Lorenz was a "'good of the species' man" [6] so accustomed to group selection thinking that he did not realize his views "contravened orthodox Darwinian theory". [6] The ethologist Niko Tinbergen praised Lorenz for his interest in the survival value of behavior, and naturalists enjoyed Lorenz's writings for the same reason. [5] In 1962, group selection was used as a popular explanation for adaptation by the zoologist V. C. Wynne-Edwards. [7] [8] In 1976, Richard Dawkins wrote a well-known book on the importance of evolution at the level of the gene or the individual, The Selfish Gene. [9]

From the mid 1960s, evolutionary biologists argued that natural selection acted primarily at the level of the individual. In 1964, John Maynard Smith, [10] C. M. Perrins (1964), [11] and George C. Williams in his 1966 book Adaptation and Natural Selection cast serious doubt on group selection as a major mechanism of evolution Williams's 1971 book Group Selection assembled writings from many authors on the same theme. [12] [13]

It was at that time generally agreed that the primary exception of social group selection was in the social insects, and the explanation was limited to the unique inheritance system (involving haplodiploidy) of the eusocial Hymenoptera such as honeybees, which encourages kin selection, since workers are closely related. [2]

Experiments from the late 1970s suggested that selection involving groups was possible. [14] Early group selection models assumed that genes acted independently, for example a gene that coded for cooperation or altruism. Genetically-based reproduction of individuals implies that, in group formation, the altruistic genes would need a way to act for the benefit of members in the group to enhance the fitness of many individuals with the same gene. [15] But it is expected from this model that individuals of the same species would compete against each other for the same resources. This would put cooperating individuals at a disadvantage, making genes for cooperation likely to be eliminated. Group selection on the level of the species is flawed because it is difficult to see how selective pressures would be applied to competing/non-cooperating individuals. [9]

Kin selection between related individuals is accepted as an explanation of altruistic behavior. R.A. Fisher in 1930 [16] and J.B.S. Haldane in 1932 [17] set out the mathematics of kin selection, with Haldane famously joking that he would willingly die for two brothers or eight cousins. [18] In this model, genetically related individuals cooperate because survival advantages to one individual also benefit kin who share some fraction of the same genes, giving a mechanism for favoring genetic selection. [19]

Inclusive fitness theory, first proposed by W. D. Hamilton in the early 1960s, gives a selection criterion for evolution of social traits when social behavior is costly to an individual organism's survival and reproduction. The criterion is that the reproductive benefit to relatives who carry the social trait, multiplied by their relatedness (the probability that they share the altruistic trait) exceeds the cost to the individual. Inclusive fitness theory is a general treatment of the statistical probabilities of social traits accruing to any other organisms likely to propagate a copy of the same social trait. Kin selection theory treats the narrower but simpler case of the benefits to close genetic relatives (or what biologists call 'kin') who may also carry and propagate the trait. A significant group of biologists support inclusive fitness as the explanation for social behavior in a wide range of species, as supported by experimental data. An article was published in Nature with over a hundred coauthors. [2]

One of the questions about kin selection is the requirement that individuals must know if other individuals are related to them, or kin recognition. Any altruistic act has to preserve similar genes. One argument given by Hamilton is that many individuals operate in "viscous" conditions, so that they live in physical proximity to relatives. Under these conditions, they can act altruistically to any other individual, and it is likely that the other individual will be related. This population structure builds a continuum between individual selection, kin selection, kin group selection and group selection without a clear boundary for each level. However, early theoretical models by D.S. Wilson et al. [20] and Taylor [21] showed that pure population viscosity cannot lead to cooperation and altruism. This is because any benefit generated by kin cooperation is exactly cancelled out by kin competition additional offspring from cooperation are eliminated by local competition. Mitteldorf and D. S. Wilson later showed that if the population is allowed to fluctuate, then local populations can temporarily store the benefit of local cooperation and promote the evolution of cooperation and altruism. [22] By assuming individual differences in adaptations, Yang further showed that the benefit of local altruism can be stored in the form of offspring quality and thus promote the evolution of altruism even if the population does not fluctuate. This is because local competition among more individuals resulting from local altruism increases the average local fitness of the individuals that survive. [23]

Another explanation for the recognition of genes for altruism is that a single trait, group reciprocal kindness, is capable of explaining the vast majority of altruism that is generally accepted as "good" by modern societies. The phenotype of altruism relies on recognition of the altruistic behavior by itself. The trait of kindness will be recognized by sufficiently intelligent and undeceived organisms in other individuals with the same trait. Moreover, the existence of such a trait predicts a tendency for kindness to unrelated organisms that are apparently kind, even if the organisms are of another species. The gene need not be exactly the same, so long as the effect or phenotype is similar. Multiple versions of the gene—or even meme—would have virtually the same effect. This explanation was given by Richard Dawkins as an analogy of a man with a green beard. Green-bearded men are imagined as tending to cooperate with each other simply by seeing a green beard, where the green beard trait is incidentally linked to the reciprocal kindness trait. [9]

Kin selection or inclusive fitness is accepted as an explanation for cooperative behavior in many species, but there are some species, including some human behavior, that are difficult to explain with only this approach. In particular, it does not seem to explain the rapid rise of human civilization. David Sloan Wilson has argued that other factors must also be considered in evolution. [24] Wilson and others have continued to develop group selection models. [25]

Early group selection models were flawed because they assumed that genes acted independently but genetically-based interactions among individuals are ubiquitous in group formation because genes must cooperate for the benefit of association in groups to enhance the fitness of group members. [15] Additionally, group selection on the level of the species is flawed because it is difficult to see how selective pressures would be applied selection in social species of groups against other groups, rather than the species entire, seems to be the level at which selective pressures are plausible. On the other hand, kin selection is accepted as an explanation of altruistic behavior. [19] [26] Some biologists argue that kin selection and multilevel selection are both needed to "obtain a complete understanding of the evolution of a social behavior system". [27]

In 1994 David Sloan Wilson and Elliott Sober argued that the case against group selection had been overstated. They considered whether groups can have functional organization in the same way as individuals, and consequently whether groups can be "vehicles" for selection. They do not posit evolution on the level of the species, but selective pressures that winnow out small groups within a species, e.g. groups of social insects or primates. Groups that cooperate better might survive and reproduce more than those that did not. Resurrected in this way, Wilson & Sober's new group selection is called multilevel selection theory. [28]

In 2010, Martin Nowak, C. E. Tarnita and E. O. Wilson argued for multi-level selection, including group selection, to correct what they saw as deficits in the explanatory power of inclusive fitness. [29] A response from 137 other evolutionary biologists argued "that their arguments are based upon a misunderstanding of evolutionary theory and a misrepresentation of the empirical literature". [2]

Wilson compared the layers of competition and evolution to nested sets of Russian matryoshka dolls. [30] The lowest level is the genes, next come the cells, then the organism level and finally the groups. The different levels function cohesively to maximize fitness, or reproductive success. The theory asserts that selection for the group level, involving competition between groups, must outweigh the individual level, involving individuals competing within a group, for a group-benefiting trait to spread. [31]

Multilevel selection theory focuses on the phenotype because it looks at the levels that selection directly acts upon. [30] For humans, social norms can be argued to reduce individual level variation and competition, thus shifting selection to the group level. The assumption is that variation between different groups is larger than variation within groups. Competition and selection can operate at all levels regardless of scale. Wilson wrote, "At all scales, there must be mechanisms that coordinate the right kinds of action and prevent disruptive forms of self-serving behavior at lower levels of social organization." [24] E. O. Wilson summarized, "In a group, selfish individuals beat altruistic individuals. But, groups of altruistic individuals beat groups of selfish individuals." [32]

Wilson ties the multilevel selection theory regarding humans to another theory, gene-culture coevolution, by acknowledging that culture seems to characterize a group-level mechanism for human groups to adapt to environmental changes. [31]

MLS theory can be used to evaluate the balance between group selection and individual selection in specific cases. [31] An experiment by William Muir compared egg productivity in hens, showing that a hyper-aggressive strain had been produced through individual selection, leading to many fatal attacks after only six generations by implication, it could be argued that group selection must have been acting to prevent this in real life. [33] Group selection has most often been postulated in humans and, notably, eusocial Hymenoptera that make cooperation a driving force of their adaptations over time and have a unique system of inheritance involving haplodiploidy that allows the colony to function as an individual while only the queen reproduces. [34]

Wilson and Sober's work revived interest in multilevel selection. In a 2005 article, [35] E. O. Wilson argued that kin selection could no longer be thought of as underlying the evolution of extreme sociality, for two reasons. First, he suggested, the argument that haplodiploid inheritance (as in the Hymenoptera) creates a strong selection pressure towards nonreproductive castes is mathematically flawed. [36] Second, eusociality no longer seems to be confined to the hymenopterans increasing numbers of highly social taxa have been found in the years since Wilson's foundational text Sociobiology: A New Synthesis was published in 1975. [37] These including a variety of insect species, as well as two rodent species (the naked mole-rat and the Damaraland mole rat). Wilson suggests the equation for Hamilton's rule: [38]

(where b represents the benefit to the recipient of altruism, c the cost to the altruist, and r their degree of relatedness) should be replaced by the more general equation

in which bk is the benefit to kin (b in the original equation) and be is the benefit accruing to the group as a whole. He then argues that, in the present state of the evidence in relation to social insects, it appears that be>rbk, so that altruism needs to be explained in terms of selection at the colony level rather than at the kin level. However, kin selection and group selection are not distinct processes, and the effects of multi-level selection are already accounted for in Hamilton's rule, rb>c, [39] provided that an expanded definition of r, not requiring Hamilton's original assumption of direct genealogical relatedness, is used, as proposed by E. O. Wilson himself. [40]

Spatial populations of predators and prey show restraint of reproduction at equilibrium, both individually and through social communication, as originally proposed by Wynne-Edwards. While these spatial populations do not have well-defined groups for group selection, the local spatial interactions of organisms in transient groups are sufficient to lead to a kind of multi-level selection. There is however as yet no evidence that these processes operate in the situations where Wynne-Edwards posited them. [41] [42]

Rauch et al.'s analysis of host-parasite evolution is broadly hostile to group selection. Specifically, the parasites do not individually moderate their transmission rather, more transmissible variants – which have a short-term but unsustainable advantage – arise, increase, and go extinct. [41]

Differing evolutionarily stable strategies Edit

The problem with group selection is that for a whole group to get a single trait, it must spread through the whole group first by regular evolution. But, as J. L. Mackie suggested, when there are many different groups, each with a different evolutionarily stable strategy, there is selection between the different strategies, since some are worse than others. [43] For example, a group where altruism was universal would indeed outcompete a group where every creature acted in its own interest, so group selection might seem feasible but a mixed group of altruists and non-altruists would be vulnerable to cheating by non-altruists within the group, so group selection would collapse. [44]

Implications in population biology Edit

Social behaviors such as altruism and group relationships can impact many aspects of population dynamics, such as intraspecific competition and interspecific interactions. In 1871, Darwin argued that group selection occurs when the benefits of cooperation or altruism between subpopulations are greater than the individual benefits of egotism within a subpopulation. [3] This supports the idea of multilevel selection, but kinship also plays an integral role because many subpopulations are composed of closely related individuals. An example of this can be found in lions, which are simultaneously cooperative and territorial. [45] Within a pride, males protect the pride from outside males, and females, who are commonly sisters, communally raise cubs and hunt. However, this cooperation seems to be density dependent. When resources are limited, group selection favors prides that work together to hunt. When prey is abundant, cooperation is no longer beneficial enough to outweigh the disadvantages of altruism, and hunting is no longer cooperative. [45]

Interactions between different species can also be affected by multilevel selection. Predator-prey relationships can also be affected. Individuals of certain monkey species howl to warn the group of the approach of a predator. [46] The evolution of this trait benefits the group by providing protection, but could be disadvantageous to the individual if the howling draws the predator's attention to them. By affecting these interspecific interactions, multilevel and kinship selection can change the population dynamics of an ecosystem. [46]

Multilevel selection attempts to explain the evolution of altruistic behavior in terms of quantitative genetics. Increased frequency or fixation of altruistic alleles can be accomplished through kin selection, in which individuals engage in altruistic behavior to promote the fitness of genetically similar individuals such as siblings. However, this can lead to inbreeding depression, [47] which typically lowers the overall fitness of a population. However, if altruism were to be selected for through an emphasis on benefit to the group as opposed to relatedness and benefit to kin, both the altruistic trait and genetic diversity could be preserved. However, relatedness should still remain a key consideration in studies of multilevel selection. Experimentally imposed multilevel selection on Japanese quail was more effective by an order of magnitude on closely related kin groups than on randomized groups of individuals. [48]

Gene-culture coevolution in humans Edit

Gene-culture coevolution (also called dual inheritance theory) is a modern hypothesis (applicable mostly to humans) that combines evolutionary biology and modern sociobiology to indicate group selection. [49] It treats culture as a separate evolutionary system that acts in parallel to the usual genetic evolution to transform human traits. [50] It is believed that this approach of combining genetic influence with cultural influence over several generations is not present in the other hypotheses such as reciprocal altruism and kin selection, making gene-culture evolution one of the strongest realistic hypotheses for group selection. Fehr provides evidence of group selection taking place in humans presently with experimentation through logic games such as prisoner's dilemma, the type of thinking that humans have developed many generations ago. [51]

Gene-culture coevolution allows humans to develop highly distinct adaptations to the local pressures and environments more quickly than with genetic evolution alone. Robert Boyd and Peter J. Richerson, two strong proponents of cultural evolution, postulate that the act of social learning, or learning in a group as done in group selection, allows human populations to accrue information over many generations. [52] This leads to cultural evolution of behaviors and technology alongside genetic evolution. Boyd and Richerson believe that the ability to collaborate evolved during the Middle Pleistocene, a million years ago, in response to a rapidly changing climate. [52]

In 2003, the behavioral scientist Herbert Gintis examined cultural evolution statistically, offering evidence that societies that promote pro-social norms have higher survival rates than societies that do not. [53] Gintis wrote that genetic and cultural evolution can work together. Genes transfer information in DNA, and cultures transfer information encoded in brains, artifacts, or documents. Language, tools, lethal weapons, fire, cooking, etc., have a long-term effect on genetics. For example, cooking led to a reduction of size of the human gut, since less digestion is needed for cooked food. Language led to a change in the human larynx and an increase in brain size. Projectile weapons led to changes in human hands and shoulders, such that humans are much better at throwing objects than the closest human relative, the chimpanzee. [54]

Behaviour Edit

In 2019, Howard Rachlin and colleagues proposed group selection of behavioural patterns, such as learned altruism, during ontogeny parallel to group selection during phylogeny. [55] [56] [57] [58]

The use of the Price equation to support group selection was challenged by van Veelen in 2012, arguing that it is based on invalid mathematical assumptions. [59]

Richard Dawkins and other advocates of the gene-centered view of evolution remain unconvinced about group selection. [60] [61] [62] In particular, Dawkins suggests that group selection fails to make an appropriate distinction between replicators and vehicles. [63]

The psychologist Steven Pinker concluded that "group selection has no useful role to play in psychology or social science", since it "is not a precise implementation of the theory of natural selection, as it is, say, in genetic algorithms or artificial life simulations. Instead it is a loose metaphor, more like the struggle among kinds of tires or telephones." [64]

The evolutionary biologist Jerry Coyne summarized the arguments in The New York Review of Books in non-technical terms as follows: [65]

Group selection isn't widely accepted by evolutionists for several reasons. First, it's not an efficient way to select for traits, like altruistic behavior, that are supposed to be detrimental to the individual but good for the group. Groups divide to form other groups much less often than organisms reproduce to form other organisms, so group selection for altruism would be unlikely to override the tendency of each group to quickly lose its altruists through natural selection favoring cheaters. Further, little evidence exists that selection on groups has promoted the evolution of any trait. Finally, other, more plausible evolutionary forces, like direct selection on individuals for reciprocal support, could have made humans prosocial. These reasons explain why only a few biologists, like [David Sloan] Wilson and E. O. Wilson (no relation), advocate group selection as the evolutionary source of cooperation. [65]


Contents

Charles Darwin was the first to discuss the concept of kin selection. In On the Origin of Species, he wrote clearly about the conundrum represented by altruistic sterile social insects that

This difficulty, though appearing insuperable, is lessened, or, as I believe, disappears, when it is remembered that selection may be applied to the family, as well as to the individual, and may thus gain the desired end. Breeders of cattle wish the flesh and fat to be well marbled together. An animal thus characterised has been slaughtered, but the breeder has gone with confidence to the same stock and has succeeded.

In this passage "the family" and "stock" stand for a kin group. These passages and others by Darwin about "kin selection" are highlighted in D.J. Futuyma's textbook of reference Evolutionary Biology [5] and in E. O. Wilson's Sociobiology. [6]

The earliest mathematically formal treatments of kin selection were by R.A. Fisher in 1930 [7] and J.B.S. Haldane in 1932 [8] and 1955. [9] J.B.S. Haldane fully grasped the basic quantities and considerations in kin selection, famously writing "I would lay down my life for two brothers or eight cousins". [10] Haldane's remark alluded to the fact that if an individual loses its life to save two siblings, four nephews, or eight cousins, it is a "fair deal" in evolutionary terms, as siblings are on average 50% identical by descent, nephews 25%, and cousins 12.5% (in a diploid population that is randomly mating and previously outbred). But Haldane also joked that he would truly die only to save more than a single identical twin of his or more than two full siblings. [11] [12] In 1955 he clarified:

Let us suppose that you carry a rare gene that affects your behaviour so that you jump into a flooded river and save a child, but you have one chance in ten of being drowned, while I do not possess the gene, and stand on the bank and watch the child drown. If the child's your own child or your brother or sister, there is an even chance that this child will also have this gene, so five genes will be saved in children for one lost in an adult. If you save a grandchild or a nephew, the advantage is only two and a half to one. If you only save a first cousin, the effect is very slight. If you try to save your first cousin once removed the population is more likely to lose this valuable gene than to gain it. … It is clear that genes making for conduct of this kind would only have a chance of spreading in rather small populations when most of the children were fairly near relatives of the man who risked his life. [13]

W. D. Hamilton, in 1963 [14] and especially in 1964 [2] [3] popularised the concept and the more thorough mathematical treatment given to it by George Price. [2] [3]

John Maynard Smith may have coined the actual term "kin selection" in 1964:

These processes I will call kin selection and group selection respectively. Kin selection has been discussed by Haldane and by Hamilton. … By kin selection I mean the evolution of characteristics which favour the survival of close relatives of the affected individual, by processes which do not require any discontinuities in the population breeding structure. [15]

Kin selection causes changes in gene frequency across generations, driven by interactions between related individuals. This dynamic forms the conceptual basis of the theory of sociobiology. Some cases of evolution by natural selection can only be understood by considering how biological relatives influence each other's fitness. Under natural selection, a gene encoding a trait that enhances the fitness of each individual carrying it should increase in frequency within the population and conversely, a gene that lowers the individual fitness of its carriers should be eliminated. However, a hypothetical gene that prompts behaviour which enhances the fitness of relatives but lowers that of the individual displaying the behaviour, may nonetheless increase in frequency, because relatives often carry the same gene. According to this principle, the enhanced fitness of relatives can at times more than compensate for the fitness loss incurred by the individuals displaying the behaviour, making kin selection possible. This is a special case of a more general model, "inclusive fitness". [16] This analysis has been challenged, [17] Wilson writing that "the foundations of the general theory of inclusive fitness based on the theory of kin selection have crumbled" [18] and that he now relies instead on the theory of eusociality and "gene-culture co-evolution" for the underlying mechanics of sociobiology.

"Kin selection" should not be confused with "group selection" according to which a genetic trait can become prevalent within a group because it benefits the group as a whole, regardless of any benefit to individual organisms. All known forms of group selection conform to the principle that an individual behaviour can be evolutionarily successful only if the genes responsible for this behaviour conform to Hamilton's Rule, and hence, on balance and in the aggregate, benefit from the behaviour. [19] [20] [21]

Formally, genes should increase in frequency when

r = the genetic relatedness of the recipient to the actor, often defined as the probability that a gene picked randomly from each at the same locus is identical by descent. B = the additional reproductive benefit gained by the recipient of the altruistic act, C = the reproductive cost to the individual performing the act.

This inequality is known as Hamilton's rule after W. D. Hamilton who in 1964 published the first formal quantitative treatment of kin selection.

The relatedness parameter (r) in Hamilton's rule was introduced in 1922 by Sewall Wright as a coefficient of relationship that gives the probability that at a random locus, the alleles there will be identical by descent. [22] Subsequent authors, including Hamilton, sometimes reformulate this with a regression, which, unlike probabilities, can be negative. A regression analysis producing statistically significant negative relationships indicates that two individuals are less genetically alike than two random ones (Hamilton 1970, Nature & Grafen 1985 Oxford Surveys in Evolutionary Biology). This has been invoked to explain the evolution of spiteful behaviour consisting of acts that result in harm, or loss of fitness, to both the actor and the recipient.

Several scientific studies have found that the kin selection model can be applied to nature. For example, in 2010 researchers used a wild population of red squirrels in Yukon, Canada to study kin selection in nature. The researchers found that surrogate mothers would adopt related orphaned squirrel pups but not unrelated orphans. The researchers calculated the cost of adoption by measuring a decrease in the survival probability of the entire litter after increasing the litter by one pup, while benefit was measured as the increased chance of survival of the orphan. The degree of relatedness of the orphan and surrogate mother for adoption to occur depended on the number of pups the surrogate mother already had in her nest, as this affected the cost of adoption. The study showed that females always adopted orphans when rB > C, but never adopted when rB < C, providing strong support for Hamilton's rule. [23]

A 2014 review of many lines of evidence for Hamilton's rule found that its predictions were strongly confirmed in a wide variety of social behaviours across a broad phylogenetic range including birds, mammals and insects, in each case comparing social and non-social taxa. [24]

Derivation Edit

Suppose we have an individual fitness function of the form

Our particular interest is in the development of altruism and when it makes sense to for altruism to flourish, relative to the costs and benefits the behavior provides. Taking the total derivative with respect to p i > , we find that:

In the linear case, this reduces to:

which is Hamilton's rule. In words, this equation implies that altruism will develop when the marginal benefit of altruism multiplied by genetic relatedness is greater than the marginal cost of altruism.

Altruism occurs where the instigating individual suffers a fitness loss while the receiving individual experiences a fitness gain. The sacrifice of one individual to help another is an example. [25]

Hamilton (1964) outlined two ways in which kin selection altruism could be favoured:

The selective advantage which makes behaviour conditional in the right sense on the discrimination of factors which correlate with the relationship of the individual concerned is therefore obvious. It may be, for instance, that in respect of a certain social action performed towards neighbours indiscriminately, an individual is only just breaking even in terms of inclusive fitness. If he could learn to recognise those of his neighbours who really were close relatives and could devote his beneficial actions to them alone an advantage to inclusive fitness would at once appear. Thus a mutation causing such discriminatory behaviour itself benefits inclusive fitness and would be selected. In fact, the individual may not need to perform any discrimination so sophisticated as we suggest here a difference in the generosity of his behaviour according to whether the situations evoking it were encountered near to, or far from, his own home might occasion an advantage of a similar kind." (1996 [1964], 51) [2]

Kin recognition: First, if individuals have the capacity to recognise kin and to discriminate (positively) on the basis of kinship, then the average relatedness of the recipients of altruism could be high enough for kin selection. Because of the facultative nature of this mechanism, kin recognition and discrimination are expected to be unimportant except among 'higher' forms of life such as the fish Neolamprologus pulcher (although there is some evidence for it among protozoa). Kin recognition may be selected for inbreeding avoidance, and little evidence indicates that 'innate' kin recognition plays a role in mediating altruism. A thought experiment on the kin recognition/discrimination distinction is the hypothetical 'green beard', where a gene for social behaviour is imagined also to cause a distinctive phenotype that can be recognised by other carriers of the gene. Due to conflicting genetic similarity in the rest of the genome, there would be selection pressure for green-beard altruistic sacrifices to be suppressed, making common ancestry the most likely form of inclusive fitness.

Viscous populations: Secondly, even indiscriminate altruism may be favoured in "viscous" populations with low rates or short ranges of dispersal. Here, social partners are typically genealogically close kin, and so altruism can flourish even in the absence of kin recognition and kin discrimination faculties—spatial proximity and circumstantial cues serving as a rudimentary form of discrimination. This suggests a rather general explanation for altruism. Directional selection always favours those with higher rates of fecundity within a certain population. Social individuals can often enhance the survival of their own kin by participating in and following the rules of their own group.

Hamilton later modified his thinking to suggest that an innate ability to recognise actual genetic relatedness was unlikely to be the dominant mediating mechanism for kin altruism:

But once again, we do not expect anything describable as an innate kin recognition adaptation, used for social behaviour other than mating, for the reasons already given in the hypothetical case of the trees. (Hamilton 1987, 425) [26]

Hamilton's later clarifications often go unnoticed, and because of the long-standing assumption that kin selection requires innate powers of kin recognition, some theorists have tried to clarify the position in recent work:

In his original papers on inclusive fitness theory, Hamilton pointed out a sufficiently high relatedness to favour altruistic behaviours could accrue in two ways — kin discrimination or limited dispersal (Hamilton, 1964, 1971,1972, 1975). There is a huge theoretical literature on the possible role of limited dispersal reviewed by Platt & Bever (2009) and West et al. (2002a), as well as experimental evolution tests of these models (Diggle et al., 2007 Griffin et al., 2004 Kümmerli et al., 2009). However, despite this, it is still sometimes claimed that kin selection requires kin discrimination (Oates & Wilson, 2001 Silk, 2002 ). Furthermore, a large number of authors appear to have implicitly or explicitly assumed that kin discrimination is the only mechanism by which altruistic behaviours can be directed towards relatives. [T]here is a huge industry of papers reinventing limited dispersal as an explanation for cooperation. The mistakes in these areas seem to stem from the incorrect assumption that kin selection or indirect fitness benefits require kin discrimination (misconception 5), despite the fact that Hamilton pointed out the potential role of limited dispersal in his earliest papers on inclusive fitness theory (Hamilton, 1964 Hamilton, 1971 Hamilton, 1972 Hamilton, 1975). (West et al. 2010, p.243 and supplement) [27]

The assumption that kin recognition must be innate, and that cue-based mediation of social cooperation based on limited dispersal and shared developmental context are not sufficient, has obscured significant progress made in applying kin selection and inclusive fitness theory to a wide variety of species, including humans, [28] [29] on the basis of cue-based mediation of social bonding and social behaviours.

Evolutionary psychologists, following early human sociobiologists' interpretation [30] of kin selection theory initially attempted to explain human altruistic behaviour through kin selection by stating that "behaviors that help a genetic relative are favored by natural selection." However, most Evolutionary psychologists recognise that this common shorthand formulation is inaccurate

[M]any misunderstandings persist. In many cases, they result from conflating "coefficient of relatedness" and "proportion of shared genes," which is a short step from the intuitively appealing—but incorrect—interpretation that "animals tend to be altruistic toward those with whom they share a lot of genes." These misunderstandings don’t just crop up occasionally they are repeated in many writings, including undergraduate psychology textbooks—most of them in the field of social psychology, within sections describing evolutionary approaches to altruism. (Park 2007, p860) [31]

As with the earlier sociobiological forays into the cross-cultural data, typical approaches are not able to find explanatory fit with the findings of ethnographers insofar that human kinship patterns are not necessarily built upon blood-ties. However, as Hamilton's later refinements of his theory make clear, it does not simply predict that genetically related individuals will inevitably recognise and engage in positive social behaviours with genetic relatives: rather, indirect context-based mechanisms may have evolved, which in historical environments have met the inclusive fitness criterion (see above section). Consideration of the demographics of the typical evolutionary environment of any species is crucial to understanding the evolution of social behaviours. As Hamilton himself puts it, "Altruistic or selfish acts are only possible when a suitable social object is available. In this sense behaviours are conditional from the start." (Hamilton 1987, 420). [26]

Under this perspective, and noting the necessity of a reliable context of interaction being available, the data on how altruism is mediated in social mammals is readily made sense of. In social mammals, primates and humans, altruistic acts that meet the kin selection criterion are typically mediated by circumstantial cues such as shared developmental environment, familiarity and social bonding. [32] That is, it is the context that mediates the development of the bonding process and the expression of the altruistic behaviours, not genetic relatedness per se. This interpretation thus is compatible with the cross-cultural ethnographic data [29] and has been called nurture kinship.

Eusociality (true sociality) is used to describe social systems with three characteristics: an overlap in generations between parents and their offspring, cooperative brood care, and specialised castes of non-reproductive individuals. [33] The social insects provide good examples of organisms with what appear to be kin selected traits. The workers of some species are sterile, a trait that would not occur if individual selection was the only process at work. The relatedness coefficient r is abnormally high between the worker sisters in a colony of Hymenoptera due to haplodiploidy. Hamilton's rule is presumed to be satisfied because the benefits in fitness for the workers are believed to exceed the costs in terms of lost reproductive opportunity, though this has never been demonstrated empirically. There are competing hypotheses, as well, which may also explain the evolution of social behaviour in such organisms. [17]

In sun-tailed monkey communities, maternal kin (kin related to by mothers) favour each other, but with relatives more distant than half-siblings this bias drops significantly. [34]

Alarm calls in ground squirrels appear to confirm kin selection. While calls may alert others of the same species to danger, they draw attention to the caller and expose it to increased risk of predation. The calls occur most frequently when the caller had relatives nearby. [35] Individual male prairie dogs followed through different stages of life modify their rate of calling when closer to kin. These behaviours show that self-sacrifice is directed towards close relatives, and that there is an indirect fitness gain. [33] Surrogate mothers adopt orphaned red squirrels in the wild only when the conditions of Hamilton's rule were met. [23]

Alan Krakauer of University of California, Berkeley has studied kin selection in the courtship behaviour of wild turkeys. Like a teenager helping her older sister prepare for a party, a subordinate turkey may help his dominant brother put on an impressive team display that is only of direct benefit to the dominant member. [36]

Even certain plants can recognise and respond to kinship ties. Using sea rocket, Susan Dudley at McMaster University, Canada compared the growth patterns of unrelated plants sharing a pot to plants from the same clone. She found that unrelated plants competed for soil nutrients by aggressive root growth. This did not occur with sibling plants. [37]

In the wood mouse (Apodemus sylvaticus), aggregates of spermatozoa form mobile trains, some of the spermatozoa undergo premature acrosome reactions that correlate to improved mobility of the mobile trains towards the female egg for fertilisation. This association is thought to proceed as a result of a "green beard effect" in which the spermatozoa perform a kin-selective altruistic act after identifying genetic similarity with the surrounding spermatozoa. [38]

Whether or not Hamilton's rule always applies, relatedness is often important for human altruism, in that humans are inclined to behave more altruistically toward kin than toward unrelated individuals. [39] Many people choose to live near relatives, exchange sizeable gifts with relatives, and favour relatives in wills in proportion to their relatedness. [39]

Experimental studies, interviews, and surveys Edit

Interviews of several hundred women in Los Angeles showed that while non-kin friends were willing to help one another, their assistance was far more likely to be reciprocal. The largest amounts of non-reciprocal help, however, were reportedly provided by kin. Additionally, more closely related kin were considered more likely sources of assistance than distant kin. [40] Similarly, several surveys of American college students found that individuals were more likely to incur the cost of assisting kin when a high probability that relatedness and benefit would be greater than cost existed. Participants’ feelings of helpfulness were stronger toward family members than non-kin. Additionally, participants were found to be most willing to help those individuals most closely related to them. Interpersonal relationships between kin in general were more supportive and less Machiavellian than those between non-kin. [41]

In one experiment, the longer participants (from both the UK and the South African Zulus) held a painful skiing position, the more money or food was presented to a given relative. Participants repeated the experiment for individuals of different relatedness (parents and siblings at r=.5, grandparents, nieces, and nephews at r=.25, etc.). The results showed that participants held the position for longer intervals the greater the degree of relatedness between themselves and those receiving the reward. [42]

Observational studies Edit

A study of food-sharing practices on the West Caroline islets of Ifaluk determined that food-sharing was more common among people from the same islet, possibly because the degree of relatedness between inhabitants of the same islet would be higher than relatedness between inhabitants of different islets. When food was shared between islets, the distance the sharer was required to travel correlated with the relatedness of the recipient—a greater distance meant that the recipient needed to be a closer relative. The relatedness of the individual and the potential inclusive fitness benefit needed to outweigh the energy cost of transporting the food over distance. [43]

Humans may use the inheritance of material goods and wealth to maximise their inclusive fitness. By providing close kin with inherited wealth, an individual may improve his or her kin's reproductive opportunities and thus increase his or her own inclusive fitness even after death. A study of a thousand wills found that the beneficiaries who received the most inheritance were generally those most closely related to the will's writer. Distant kin received proportionally less inheritance, with the least amount of inheritance going to non-kin. [44]

A study of childcare practices among Canadian women found that respondents with children provide childcare reciprocally with non-kin. The cost of caring for non-kin was balanced by the benefit a woman received—having her own offspring cared for in return. However, respondents without children were significantly more likely to offer childcare to kin. For individuals without their own offspring, the inclusive fitness benefits of providing care to closely related children might outweigh the time and energy costs of childcare. [45]

Family investment in offspring among black South African households also appears consistent with an inclusive fitness model. [46] A higher degree of relatedness between children and their caregivers frequently correlated with a higher degree of investment in the children, with more food, health care, and clothing being provided. Relatedness between the child and the rest of the household also positively associated with the regularity of a child's visits to local medical practitioners and with the highest grade the child had completed in school. Additionally, relatedness negatively associated with a child's being behind in school for his or her age.

Observation of the Dolgan hunter-gatherers of northern Russia suggested that, while reciprocal food-sharing occurs between both kin and non-kin, there are larger and more frequent asymmetrical transfers of food to kin. Kin are also more likely to be welcomed to non-reciprocal meals, while non-kin are discouraged from attending. Finally, even when reciprocal food-sharing occurs between families, these families are often very closely related, and the primary beneficiaries are the offspring. [47]

Other research indicates that violence in families is more likely to occur when step-parents are present and that "genetic relationship is associated with a softening of conflict, and people's evident valuations of themselves and of others are systematically related to the parties' reproductive values". [48]

Numerous other studies suggest how inclusive fitness may work amongst different peoples, such as the Ye’kwana of southern Venezuela, the Gypsies of Hungary, and the doomed Donner Party of the United States. [49] [50] [51] [52] [53]

Vervet monkeys display kin selection between siblings, mothers and offspring, and grandparent-grandchild. These monkeys utilise allomothering, where the allomother is typically an older female sibling or a grandmother. Other studies have shown that individuals will act aggressively toward other individuals that were aggressive toward their relatives. [54] [55]

Synalpheus regalis is a eusocial shrimp that protects juveniles in the colony. By defending the young, the large defender shrimp can increase its inclusive fitness. Allozyme data revealed that relatedness within colonies is high, averaging 0.50, indicating that colonies in this species represent close kin groups. [56]

Though originally thought unique to the animal kingdom, evidence of kin selection has been identified in the plant kingdom. [57]

Competition for resources between developing zygotes in plant ovaries increases when seeds are sired by different fathers. [58] How developing zygotes differentiate between full siblings and half-siblings in the ovary is undetermined, but genetic interactions are thought to play a role. [58] Nonetheless, competition between zygotes in the ovary is detrimental to the reproductive success of the mother plant, as fewer zygotes mature into seeds, and is also thought to harm the mother plant itself. [58] As such, the reproductive traits and behaviors of plants suggests the evolution of behaviors and characteristics that increase the genetic relatedness of fertilized eggs in the plant ovary, thereby fostering kin selection and cooperation among the seeds as they develop. These traits differ among plant species. Some species have evolved to have fewer ovules per ovary, commonly one ovule per ovary, thereby decreasing the chance of developing multiple, differently fathered seeds within the same ovary. [58] Multi-ovulated plants have developed mechanisms that increase the chances of all ovules within the ovary being fathered by the same parent. Such mechanisms include dispersal of pollen in aggregated packets and closure of the stigmatic lobes after pollen is introduced. [58] The aggregated pollen packet releases pollen gametes in the ovary, thereby increasing likelihood that all ovules are fertilized by pollen from the same parent. [58] Likewise, the closure of the ovary pore prevents entry of new pollen. [58] Other multi-ovulated plants have evolved mechanisms that mimic the evolutionary adaption of single-ovulated ovaries the ovules are fertilized by pollen from different individuals, but the mother ovary then selectively aborts fertilized ovules, either at the zygotic or embryonic stage. [58]

After seeds are dispersed, kin recognition and cooperation affects root formation in developing plants. [59] Studies have found that the total root mass developed by Ipomoea hederacea (morning glory shrubs) grown next to kin is significantly smaller than those grown next to non-kin [59] [60] shrubs grown next to kin thus allocate less energy and resources to growing the larger root systems needed for competitive growth. Interestingly, when seedlings were grown in individual pots placed next to kin or non-kin relatives, no difference in root growth was observed. [60] This indicates that kin recognition occurs via signals received by the roots. [60]

Groups of I. hederacea plants also display greater variation in height when grown with kin than when grown with non-kin. [59] The evolutionary benefit provided by this was further investigated by researchers at the Université de Montpellier. They found that the alternating heights seen in kin-grouped crops allowed for optimal light availability to all plants in the group shorter plants next to taller plants had access to more light than those surrounded by plants of similar height. [61]

The above examples illustrate the effect of kin selection in the equitable allocation of light, nutrients, and water. The evolutionary emergence of single-ovulated ovaries in plants has eliminated the need for a developing seed to compete for nutrients, thus increasing its chance of survival and germination. [58] Likewise, the fathering of all ovules in multi-ovulated ovaries by one father, decreases the likelihood of competition between developing seeds, thereby also increasing the seeds' chances of survival and germination. [58] The decreased root growth in plants grown with kin increases the amount of energy available for reproduction plants grown with kin produced more seeds than those grown with non-kin. [59] [60] Similarly, the increase in light made available by alternating heights in groups of related plants is associated with higher fecundity. [59] [61]

Kin selection has also been observed in plant responses to herbivory. In an experiment done by Richard Karban et al, leaves of potted Artemisia tridentata (sagebrushes) were clipped with scissors to simulate herbivory. The gaseous volatiles emitted by the clipped leaves were captured in a plastic bag. When these volatiles were transferred to leaves of a closely related sagebrush, the recipient experienced lower levels of herbivory than those that had been exposed to volatiles released by non-kin plants. [57] Sagebrushes do not uniformly emit the same volatiles in response to herbivory: the chemical ratios and composition of emitted volatiles vary from one sagebrush to another. [57] [62] Closely related sagebrushes emit similar volatiles, and the similarities decrease as relatedness decreases. [57] This suggests that the composition of volatile gasses plays a role in kin selection among plants. Volatiles from a distantly related plant are less likely to induce a protective response against herbivory in a neighboring plant, than volatiles from a closely related plant. [57] This fosters kin selection, as the volatiles emitted by a plant will activate the herbivorous defense response in related plants only, thus increasing their chance of survival and reproduction. [57]

Mechanisms of kin selection in plants Edit

The ability to differentiate between kin and non-kin is not necessary for kin selection in many animals. [63] However, because plants do not reliably germinate in close proximity to kin, it is thought that, within the plant kingdom, kin recognition is especially important for kin selection. [63] Unfortunately, the mechanisms by which kin recognition occurs in plants remain unknown. [63] [64] Below are some hypothesized processes involved in kin recognition.

  • Communication through Roots: Plants are thought to recognize kin through the secretion and reception of root exudates. [63][65][66] It has been hypothesized that exudates actively secreted by roots of a plant are detected by roots of neighboring plants. [65][66] However the root receptors responsible for recognition of kin exudates, and the pathway induced by receptor activation, remain unknown. [66] The mycorrhizae associated with roots might facilitate reception of exudates secreted by neighboring plants, but the mechanism through which this may occurs is also unknown. [67]
  • Communication through Volatiles: In Karban et al's study of kin recognition in Artemisia tridentata (described in the section above), the volatile-donating sagebrushes were kept in individual pots, separate from the plants that received the volatiles. [57] This suggests that root signaling is either not involved in, or not necessary to induce a protective response against herbivory in neighboring kin plants. [57] Karban et al hypothesize that plants may be able to differentiate between kin and non-kin based on the composition of volatiles emitted by neighboring plants. [57] Because the donor volatiles were only exposed to the recipient sagebrush's leaves, [57] it is likely the volatiles activate a receptor protein in the plant's leaves. The identity of this receptor, and the signaling pathway triggered by its activation, both remain to be discovered [68]

The theory of kin selection has been criticised by Alonso in 1998 [69] and by Alonso and Schuck-Paim in 2002. [70] Alonso and Schuck-Paim argue that the behaviours which kin selection attempts to explain are not altruistic (in pure Darwinian terms) because: (1) they may directly favour the performer as an individual aiming to maximise its progeny (so the behaviours can be explained as ordinary individual selection) (2) these behaviours benefit the group (so they can be explained as group selection) or (3) they are by-products of a developmental system of many "individuals" performing different tasks (like a colony of bees, or the cells of multicellular organisms, which are the focus of selection). They also argue that the genes involved in sex ratio conflicts could be treated as "parasites" of (already established) social colonies, not as their "promoters", and, therefore the sex ratio in colonies would be irrelevant to the transition to eusociality. [69] [70] Those ideas were mostly ignored until they were put forward again in a series of papers by E. O. Wilson, Bert Hölldobler, Martin Nowak and others. [71] [72] [73] Nowak, Tarnita and Wilson argued that

Inclusive fitness theory is not a simplification over the standard approach. It is an alternative accounting method, but one that works only in a very limited domain. Whenever inclusive fitness does work, the results are identical to those of the standard approach. Inclusive fitness theory is an unnecessary detour, which does not provide additional insight or information.

They, like Alonso (1998) and Alonso and Schuck-Paim (2002) earlier, argue for a multi-level selection model instead. [17] This aroused a strong response, including a rebuttal published in Nature from over a hundred researchers. [74]


Stabilizing Selection

The most common of the types of natural selection is stabilizing selection. In stabilizing selection, the median phenotype is the one selected for during natural selection. This does not skew the bell curve in any way. Instead, it makes the peak of the bell curve even higher than what would be considered normal.

Stabilizing selection is the type of natural selection that human skin color follows. Most humans are not extremely light skinned or extremely dark skinned. The majority of the species fall somewhere in the middle of those two extremes. This creates a very large peak right in the middle of the bell curve. This is usually caused by a blending of traits through incomplete or codominance of the alleles.


Smallest unit on which selection can act - Biology

Mechanisms: the processes of evolution

Evolution is the process by which modern organisms have descended from ancient ancestors. Evolution is responsible for both the remarkable similarities we see across all life and the amazing diversity of that life — but exactly how does it work?

Fundamental to the process is genetic variation upon which selective forces can act in order for evolution to occur. This section examines the mechanisms of evolution focusing on:

    and the genetic differences that are heritable and passed on to the next generation

Mutation, migration (gene flow), genetic drift, and natural selection as mechanisms of change

The random nature of genetic drift and the effects of a reduction in genetic variation

How variation, differential reproduction, and heredity result in evolution by natural selection and

How different species can affect each other's evolution through coevolution.


Tissues are groups of cells with both a shared structure and function. Cells that make up animal tissues are sometimes woven together with extracellular fibers and are occasionally held together by a sticky substance that coats the cells. Different types of tissues can also be arranged together to form organs. Groups of organs can in turn form organ systems.

Cells within the human body have different life spans based on the type and function of the cell. They can live anywhere from a few days to a year. Certain cells of the digestive tract live for only a few days, while some immune system cells can live for up to six weeks. Pancreatic cells can live for as long as a year.


The History of the Cell Theory

The cell theory and ideas about cells and living things evolved over several centuries. Here are the key dates for the cell theory:

1665: Robert Hooke is the first person to observe cells when he looks at a slice of cork in a microscope.

1665: Francesco Redi disproves spontaneous generation by showing maggots will only grow on uncovered meat, not meat enclosed in a jar. His work later contributes to part three of the cell theory.

1670s: Antonie van Leeuwenhoek, a Dutch scientist, begins his work developing better microscopes that allow scientists to see cells and the organelles they contain more clearly.

1839: German scientists Matthias Schleiden and Theodor Schwann describe the first two parts of the cell theory. Schleiden stated that all plants are made up of cells, while Schwann stated all animals are made up of cells. Schleiden and Schwann are generally credited as the developers of cell theory.

1855: Rudolf Virchow, another German scientist, describes the third part of cell theory, that all cells come from existing cells.

Since then, microscopes have continued to become more and more refined, making it possible to study cells even more closely and allowing scientists to expand on the original cell theory.


The 3 Types of Natural Selection

Natural selection is defined as a process or a “force” that allows for organisms better adapted to their environment to better survive and produce more offspring. The theory of natural selection was first founded by Charles Darwin. The process of natural selection is important and is a driving force for evolution. For organisms to evolve, there needs to be differences in traits between organisms that provide certain advantages or disadvantages, and it is these traits that natural selection acts upon.

When it comes to natural selection, there are three different types of selection that can occur. These types include the following:

Stabilizing Selection

This type of natural selection occurs when there are selective pressures working against two extremes of a trait and therefore the intermediate or “middle” trait is selected for. If we look at a distribution of traits in the population, it is noticeable that a standard distribution is followed:

Example: For a plant, the plants that are very tall are exposed to more wind and are at risk of being blown over. The plants that are very short fail to get enough sunlight to prosper. Therefore, the plants that are a middle height between the two get both enough sunlight and protection from the wind.

Directional Selection

This type of natural selection occurs when selective pressures are working in favour of one extreme of a trait. Therefore when looking at a distribution of traits in a population, a graph tends to lean more to one side:

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Example: Giraffes with the longest necks are able to reach more leaves to each. Selective pressures will work in the advantage of the longer neck giraffes and therefore the distribution of the trait within the population will shift towards the longer neck trait.

Disruptive Selection

This type of natural selection occurs when selective pressures are working in favour of the two extremes and against the intermediate trait. This type of selection is not as common. When looking at a trait distribution, there are two higher peaks on both ends with a minimum in the middle as such:

Example: An area that has black, white and grey bunnies contains both black and white rocks. Both the traits for white and black will be favored by natural selection since they both prove useful for camouflage. The intermediate trait of grey does not prove as useful and therefore selective pressures act against the trait.

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Smallest unit on which selection can act - Biology

For natural selection to proceed there must be heritable variation in phenotypes and the variation in phenotype must be associated with differential survival and/or reproduction, i.e., there must be differential fitness . By inference then, any entities exhibiting heritable variation in rates of reproduction can evolve . We need not restrict our thinking to "individuals" in "populations" in the traditional senses of these words.

Nature is organized in a hierarchical fashion. In terms of entities that can be heritable we can consider genes, chromosomes, genomes, individuals, groups, demes, populations, species , etc. Each of these entities meets the requirements of units that can be acted upon by selection. At which level(s) does selection act? Answer: all of them. What then is the important unit of selection? Answer: it depends.

First, some historical context. Serious consideration of a unit of selection other than the individual was advanced by V. C. Wynne-Edwards (1962, Animal Dispersion in Relation to Social Behavior ). Populations have their own rates of origination and extinction and selection could thus operate at the level of the group. Idea based on observation that many species tend to curb their reproductive rate/output when population densities are high. This behavior would favor groups that exhibited the behavior and select against those that did not i.e., there would be group selection .

G. C. Williams responded to this idea with Adaptation and Natural Selection (1966) arguing that this behavior would be less fit than a cheating behavior where individuals did not reduce their reproductive output at times of high density/low food availability. In general selection at the level of the individual would be much stronger than selection at the level of groups. In keeping with Williams' claim that one should always seek the simplest explanation for selective/adaptive explanations, individual selection is usually sufficient to account for patterns.

Group selectionist thinking leads to statements such as "good for the species" when it is entirely likely that it may be good for the individual as well: reduced reproductive effort in times of low food may increase an individual's reproductive output at a later date.

Examples of selection at different hierarchical levels: Genic selection is selection at the gene level best example is meiotic drive (segregation distortion) where one gametic type (often one chromosomal type) is transmitted into the gamete pool (or next generation) in excess (or deficiency). The T locus in mice: affects tail length but also viability. TT homozygotes have normal long tails Tt heterozygotes have short tails and transmit

90% of the t allele to their sperm tt homozygotes are sterile. Meiotic drive will increase frequency of t allele to point where that become frequent enough to occur as tt homozygotes with appreciable frequency, whereupon selection works against t alleles. Opposite Selection at two levels selection for at the level of the gene against at the level of the genotype (organism).

In this system the balance of opposing selection coefficients at different levels should give an equilibrium allele frequency of 0.7 for f(t) allele (using data not provided here). In nature f(t) = 0.36. Discrepancy due to small local groups and drift . Some local breeding groups (2 - 4 individuals) fixed for the t allele and since tt is sterile, these demes go extinct reducing the f(t). Thus we have selection at three levels : genic, individual and intergroup all contributing to the maintenance of the t/T polymorphism.

Would we expect to detect meiotic drive systems in natural populations? If a new mutation arose that introduced a bias in the transmission of the chromosome on which it was located, then it would sweep to fixation and the locus would be homozygous for the "drive" allele. Meiotic drive can only be detected in heterozygous state, so the drive system would disappear when the drive allele went to fixation. There will be a window of time where the allele is increasing in frequency, but this could be short-lived. If the drive allele reduced viability in the homozygous state (as the T locus example), then variation can be maintained and the drive system would persist longer, making it more likely to be detected.

Another case where genic selection may act: sex ratios. Why should the sex ratio be 1:1 in most diploid species? Assume a sex ratio of 40% males and 60% females. Males in this case are in limited supply. Any gene leading to the production of more males (a allele results in more males the A allele at a sex determination locus) will be favored until the frequency of males is >50%. Sex ratios tend to stabilize at 50:50 (R. A. Fisher, 1930).

Kin selection and altruistic behavior many species of animals that live in groups give warning calls which alert other individuals about predators, etc. How could this behavior evolve when making the call alerts the predator to the callers location and increases the possibility of the caller becoming prey. Mammals that nurse their young: major energy investment for the mother may be thought to reduce her fitness, but make obvious sense sine the individuals that benefit are close relatives (offspring). Put in fitness terms: how could a trait evolve that lowers individual fitness?

Key point is the term individual . If the ones that benefit from the behavior are related , the loss in individual fitness may be regained in inclusive fitness , i.e., individual fitness plus fitnesses of relatives . W. D. Hamilton argued that an altruistic trait could evolve if cost to altruist/benefit to recipient < genetic relatedness ( C/B < r ) where r is an estimate of the probability of the donor and recipient having allele identical by descent. For example parent - offspring have r=0.5 siblings have r=0.5 grandparent-grandchild have r=0.25.

Idea of inclusive fitness implies that one's fitness is determined by one's own life time reproductive output and the reproductive output of relatives, scaled by their degree of relatedness (r). In the warning call example, if calling out to warn about the arrival of a hawk killed you but saved three reproductively active siblings, it would have been worth it. If it only saved two siblings, it probability wasn't worth it. Obviously one cannot tabulate the payoff of event x, y, or z. The point is that the notion of inclusive fitness provides a fitness context where altruistic behavior could evolve even when it appears to decrease individual fitness. Two modes of selection : individual selection opposes altruism selection among kin groups favors altruism.

Classic examples: helpers at the nest in birds. Young offspring remain at the nest to help their parents produce more siblings in subsequent years/seasons. Helpers may contribute more to their own fitness by aiding in the production of siblings than by trying to reproduce themselves and failing due to lack of experience or availability of nest sites. Sterile workers in hymenoptera (ants, bees, wasps): males are haploid (develop from unfertilized egg) so sisters have r=0.75 because male contributes the same allele (relatedness between sibs for paternal genes = 1.0 relatedness between sibs for maternal genes = 0.5 among diploid female workers this averages out to r=0.75). A female worker does more to propagate her own genes by staying in the nest and aiding in the production of sisters (r=0.75) than by going off and producing her own daughters (r=0.5). Used as an explanation for the evolution of sociality (e.g., colonies ) in hymenoptera.

Group selection = variation in the rate of increase or extinction among groups as a function of their genetic composition. Again consider how an altruistic trait could increase in frequency. Differential rates of extinction : allele A confers altruistic behavior at a selective disadvantage to allele a within the group. Should lead to the reduction of allele A. But groups with high frequency of A may be less likely to go extinct (due to better exploitation of resources). Over all groups with high frequency of A persist and f(A) increases.

Differential productivity : similar to model above but altruistic trait affects reproductive output of group. Selection against A allele within groups (selfish types have higher short term fitness) but groups with high frequency of A exploit resources more prudently and actually produce more offspring over the long term: f(A) increases. Model this as follows: assume a haploid trait with A=altruist, a=selfish p=f(A). In each population p decreases within a population through time due to selfish individuals out competing altruistic individuals. But in all populations as a whole the altruist gene increases over time. Below, the average f(A) across all populations is 0.5 at the start:

Clearly, if this system were to continue for many generations, the frequency of the altruist gene would decrease within each population. But under conditions where the selection favoring selfish genes was weak and the group selection increasing the probability of staying extant (or the growth rate of the population) was strong, and altruist allele might be preserved. Because the conditions are so restrictive, group selection is presumed to be a rare phenomenon.

Group selection often involves plausible models but require that interdeme (group) selection be strong . Would have to be very strong to overcome selection among individuals within populations. Other complicating factors: turnover rate of individuals is faster than of populations/groups fixation of less fit allele is unstable to invasion by new mutant allele or "selfish" allele introduced by gene flow. New research on multilevel selection suggests that there should not be the necessary association between altruism and "sacrifice" or genetic "suicide". Cooperation among individuals can actually result in higher group fitness without the assumed loss of individual fitness (see a meeting review in Science (9 August 1996) vol 273:739-740). D. S. Wilson makes the analogy between the optimal clutch size argument of D. Lack and the optimal group of Wynne-Edwards. With too many eggs in a clutch an individual may die trying to support them all, so some intermediate clutch size is "optimal" (see Life History Lecture). Optimal groups may evolve intermediate density by the same trade-off mechanism.

A further problem for group selection: with localize population structure, there can be considerable inbreeding which increases relatedness (r). Thus inter "group" selection that gives the appearance of evolution of altruistic traits may be mediated by kin selection due to the high relatedness among individuals.

Later we will consider species selection . Some lineages have more species than others, but are these lineages more fit? Is this simply a pattern (more species) or is it really a different process ? Is it simple like bacteria in chemostats: a higher birth/death ratio some lineages seem to speciate faster than their members go extinct? Is this mediated at the level of the species, or can we explain it (as G. C. Williams might like) at the level of individuals within populations?

Richard Dawkins likes to couch this discussion in terms of replicators and vehicles . Replicators are any entities of which copies are made selection will favor replicators with the highest replication rate. Vehicles are survival machines: organisms are vehicles for replicators and selection will favor vehicles that are better at propagating the replicators that reside within them. There is a hierarchy of both replicators and vehicles. The key issues are that 1) the "unit" of selection is one that is potentially immortal : organisms die, but their genes could be passed on indefinitely. The heritability of a gene is greater than that of a chromosome is > that of a cell > organism > and so on. But , because of linkage we should not think of individual genes as the units it is the stretch of chromosome upon which selection can select, given certain rates of recombination. Issue 2) is that selection acts on phenotypes that are the product of the replicators, not on the replicators themselves, but the vehicles have lower heritability and immortality than replicators. What then is the unit of selection?? All of them, just of different strengths and effects at different levels.


Watch the video: Speech Act Theory (August 2022).