Population genetics is the branch of genetics whose objective is to describe the variation and distribution of allelic frequency to explain evolutionary phenomena, and so it is definitively seated within the field of evolutionary biology. To do this, it defines a population as a group of individuals of the same species that are reproductively isolated from other related groups, in other words it is a group of organisms that share the same habitat and reproduce among themselves. These populations are subject to evolutionary changes underlying genetic changes, which in turn are influenced by factors such as natural selection, genetic drift, gene flow, mutation and genetic recombination.
Population genetics is thus an essential element of modern evolutionary synthesis. Its main founders, Sewall Wright, J.B.S. Haldane and Ronald Fisher, also laid the formal foundations of quantitative genetics. The founding works of population genetics are The Genetical Theory of Natural Selection (Fisher 1930), Evolution in Mendelian Populations (Wright 1931) and The Causes of Evolution (Haldane 1932). While at first it was a discipline highly based on mathematical analysis, modern population genetics includes contributions based on theoretical, practical, and field work. The processing of computer data, thanks to the theory of coalescence, has allowed the advancement of this field since the 1980s.
Population genetics began as a reconciliation of models of Mendel’s laws and biostatistics. A key step was the contribution of the British biologist Ronald Fisher. In a series of papers beginning in 1918 and culminating in his 1930 book The Genetical Theory of Natural Selection, in which Fisher showed that continuous variation measured by biostatistics could be produced by the combined actions of many discrete genes, and that natural selection could change the allelic frequency in a population, resulting in the evolution of the population. In a series of papers that began in 1924, another British geneticist, J.B.S. Haldane, solved the mathematics of changes in the allelic frequencies of the locus of a singular gene and under a wide range of conditions. Haldane also used a statistical analysis with examples of the real world of natural selection, such as the evolution of industrial melanism. Wright showed that the selection coefficients could be larger than Fisher had assumed, leading to a faster adaptive evolution.
American biologist Sewall Wright, with a background in animal breeding experiments, focused on combinations of genes that interacted and on the effects of inbreeding in populations that are small and relatively isolated and that exhibit genetic drift, always more likely in small populations. In 1932, Wright introduced the concept of an adaptable landscape. He argued that genetic drift and inbreeding could result in a small, isolated population moving away from an adaptive peak and that natural selection led the population to different adaptive peaks.
Fisher, Haldane and Wright are considered to be the founders of the discipline of population genetics. Natural selection was integrated with Mendel’s laws, the first and critical step in the development of a unified theory of how evolution works. John Maynard Smith was a student of Haldane, while Fisher’s writings inspired William Donald Hamilton. The American George R. Price worked with Hamilton and Maynard Smith. Wright greatly influenced the American Richard Lewontin and the Japanese Motoo Kimura.
Selection against genetic drift
Fisher and Wright had some fundamental disagreements about the role of selection and genetic drift.
The British biologist E.B. Ford, the pioneer of ecological genetics, demonstrated continuously during the 1930s and 1940s the power of selection due to ecological factors, which included the ability to maintain genetic diversity through polymorphism, e.g. blood group in humans. The work Ford did, in collaboration with Fisher, contributed to modern evolutionary synthesis, which emphasized natural selection rather than genetic drift.
However, recent studies of eukaryota transposons, and their impact on speciation, point to a central role for processes that are not adaptable, such as mutation and genetic drift, which are also seen as major factors in the evolution of genome complexity.
Natural selection is the process by which certain characteristics of an individual make its survival and reproduction more likely. Natural selection acts on phenotypes, or the observable characteristics of organisms, but the hereditary genetic basis of any phenotype that gives a reproductive advantage will become more common in the population.
Genetic drift is the change in the allelic frequency of species as a stochastic effect of random sampling on reproduction and the loss of alleles by chance. Children’s alleles are a random sampling of parents’ alleles. Changes in genetic drift are not a consequence of natural selection, and may be beneficial, neutral, or negative for reproduction and survival. The alleles of children are a random sampling of parents’ alleles.
When there are many copies of an allele in a population, the effect of genetic drift is less than when alleles are presented in fewer copies. Scientists have had vigorous debates about the relative importance of genetic drift compared to natural selection. Ronald Fisher said that genetic drift only plays a minor role in evolution, a view that was dominant for several decades. In 1968 Motoo Kimura revived the debate with the neutralist theory of molecular evolution, which says that neutral mutations and genetic drift cause most changes in genetic matter. John Hillespie has criticized the conjecture of the active involvement of genetic drift through sampling errors in evolution. Will Provine also argued, that selection at linked sites is a stochastic force.
The genetic population of the genetic drift is described either by using branching processes or by using a diffusion equation that describes changes in allelic frequency. These processes are usually addressed by the population genetics models of Wright-Fisher and John Moran.
Mutations are the main sources of genetic variability in the form of new alleles. They can result in several types of changes in DNA sequences; these changes can have either neutral, positive (altering the gene product) or negative (preventing the gene from functioning) effects. Studies of the fly Drosophila melanogaster suggest that if a gene has modifications on a protein product there is a 70% probability that this is harmful, with the remaining 30% being neutral or of little benefit to the organism. Mutations may involve large sections of DNA that, being duplicated by recombination processes, are delineated or duplicated. Another possibility is that point mutations insert or delect loose nucleotides or small sequences.
These duplications are a major source of raw materials that form the evolution of new genes, with tens to hundreds of genes duplicating in this way every million years. Most of these genes are parts of larger gene families made from homologous genes with shared ancestors. Several methods are used to produce novel genes, the most common being duplication, mutation of an ancestral gene, and recombination of parts of different genes to form new combinations with new functions. Protein domains act as modules, each with a particular and independent function that can be mixed together to produce genes that encode new proteins with novel properties. For example, the human eye uses four genes to make structures that feel light: three cone genes for color perception and a stick gene for night vision; all four came from a single ancestral gene. The human eye uses four genes to make structures that feel light: three cone genes for color perception and one stick gene for night vision; all four came from a single ancestral gene.
Genetic flow or migration is the transfer of alleles of genes from one population to another thanks to different factors such as mobility. It usually occurs between the same species, forming hybrids when the opposite is the case (the process of gene flow between species is called horizontal transfer).
It should be noted that the loss of genetic variability in populations brings with it two serious problems:
It limits the possibility that man can make genetic improvement in the species.
It diminishes the biological efficiency (fitness) of the species before new environmental changes.
For its part, the presence of genetic variability is desirable not only for genetic improvement or conservation of species, since the fundamental role of genetic variability is to be the raw material for evolutionary processes, without variability there is no evolution. The interaction of these factors with populations in time, allows the existence of large numbers of species with varied population structures and life forms.
Basic population genetics models consider only one gene at a time. In practice, epistasia and linking locis can be very important.
Epistasia is the interaction that arises between different genes when expressing a certain phenotypic character due to the action that one or several genes exert on the action of a specific gene. The gene whose phenotype is being expressed is called epistatic, while the suppressed or altered gene is called hypostatic.
Ligation is the physical association between two locis, i.e. their proximity in the same strand of DNA negatively affects their frequency of recombination between them during meiosis, and thus increases the likelihood of a joint genetic inheritance. Therefore recombination breaks this linkage very slowly, being a problem for population genetic models that treat one locus of the gene at a time. This characteristic can, however, be exploited as a method for detecting the action of natural selection through selective sweeps.