From 1900 until about 1920, adherents of Mendelian genetics believed that only mutations were responsible for evolution and that natural selection was not important in this process.
After that several scientists began to reconcile the ideas of natural selection with the facts of genetics, This culminated in the formulation of the synthetic theory of evolution, sometimes also called Neodarwinism.
As Darwin had already proposed, this theory considers population as the evolutionary unit. A population can be defined as a group of individuals of the same species that occur in the same geographical area, in the same time interval.
Each population has a certain gene set, which can be altered according to evolutionary factors. The gene pool of a population is the pool of all genes present in that population. Thus, the larger the gene pool of the population, the greater the genetic variability.
The main evolutionary factors that affect the population's gene pool can be grouped into two categories:
- factors that tend to increase population genetic variability - mutation and permutation;
- factors that act on the already established genetic variability - migration, genetic drift and natural selection.
It is known that a population is evolving when changes in the frequency of its genes occur. Evolution is now considered the central and unifying concept of biology, and a striking phrase that emphasizes this idea was written by scientist Dobzhansky:Nothing is done in biology except in light of evolution.”.
Genetic basis of evolution
Mutation creates new genes, and recombination mixes them with existing genes, originating genetically varied individuals from a population. Natural selection, in turn, favors carriers of certain adaptive gene sets, which tend to survive and reproduce on a larger scale than others. Due to the performance of these and other evolutionary factors, the genetic composition of populations changes over time.
Mutations can be chromosomal or gene. Chromosomal mutations can be changes in the number or shape of chromosomes. Gene mutations originate from changes in the sequence of nitrogenous bases of a given gene during duplication of the DNA molecule. Such alteration may occur by nucleotide loss, addition or substitution, which may give rise to a gene capable of encoding a different protein than should have been encoded.
Gene mutations are considered the primary sources of variability because they increase the number of alleles available in a locus, increasing a gene pool of the population. Although they occur spontaneously, they can be caused by mutagenic agents such as radiation and certain chemicals (the illegal drug LSD, for example).
Mutations do not occur to adapt the individual to the environment. They occur at random and, by natural selection, are maintained when adaptive (positive selection) or otherwise eliminated (negative selection). They may occur in somatic cells or germ cells; In the latter case, mutations are of fundamental importance to evolution, as they are transmitted to offspring.
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