Population genetics is the study of the inheritance, distribution, and fluctuation of allele frequencies as affected by the four main forces of evolutionary genomics: natural selection, mutation, genetic drift, and migration. Population genetics is the overarching discipline of which quantitiative genetics is a crucial subfield that calculates selective effects. Ecological genetics is another subfield that closely examines gene-environment interactions, particularly in wild populations. Molecular genetics deals with the structure and function of genes at a molecular level. Thus, quantitative and ecological genetics are cognate areas of population genetics, and taken together, they all extend classical genetics in unique ways.
Genetic variation is typically cited as the ultimate fuel of evolution, but as organisms adapt in constantly changing environs, it is more precisely allelic variation upon which operate the forces of evolution. Alleles are variations of the same genetic loci that can express different phenotypes. Novel alleles arise in genomic populations via random mutation, and the frequency of occurrence of an allele changes regularly as a result of mutation, genetic drift, and selection. Such study of genetic frequency is relevant to a wide range of issues to do with diploid genomic populations. All diploid organisms have two elements—one allele inherited from each parent—at each genetic locus. Where these two elements are different, the organism is said to be heterozygous. Mendelian principles allowed early population geneticists to study heritable form of allelic traits, that is, the genotype, with an emphasis on the observable phenotype: the outward, expressed traits or physical manifestation of specific genetic loci, including variant alleles and heterozygotes.
Meanwhile, in 1908, the mathematical foundation of population genetics was independently discovered by the British mathematician G. A. Hardy and a German physician, W. Weinberg. The Hardy-Weinberg formula establishes parameters by which to predict expected genotypic frequencies of alleles in diploid populations. In a large, randomly breeding population, intergenerational allelic frequencies do not change in the absence of unbalanced mutation, gene migration, selection, or genetic drift. This equilibrium is a mathematically simple expression (for example, p2 + 2pq + q2 = 1). Thus can be calculated how alleles evolve in serial generations to the extent that they vary from equilibrium parameters.
Variations of allelic frequency are quantified with respect to demographic adjustments of population structures in evolutionary environmental space and time. Hence, population genetics is the systematic elucidation of essential biological phenomena, such as adaptation and speciation—with adaptation here meaning enhanced reproductive success of a given anatomical, physiological, or behavioral feature evident by increased allelic frequency over successive generations and speciation meaning the appearance of qualitatively novel species of organism. A key premise of population genetics is that inheritance of both quantitative and qualitative traits are based on similar natural laws.
Population genetics is the main technique derived from the so-called modern evolutionary synthesis of the early 20th century, which melded neo-Darwinism with the rediscovery of Mendel’s elegant laws of genetics. It is neo-Darwinian in that Darwin could not have known the precise basis of biological inheritance as the mechanism of genetics. As a consequence, Darwin never fully rejected the Lamarckian notion of acquired-but-heritable traits. However, his intellectual heir, George John Romanes, dismissed such Lamarckian ideas and termed evolutionary mechanisms that were necessary, sufficient, and yet entirely epigenetic neo-Darwinism.
Yet only with the rediscovery of Mendelian genetics in 1900 was a neo-Darwinian evolutionary science made more complete and rigorous. Still, it was not yet immediately clear on what variations processes of natural selection operated most directly. A biometric approach was formulated by the pioneer statistician Karl Pearson, who also widely influenced modern medicine, epidemiology, and anthropology. Pearson focused study on the importance of subtle heritable differences as the basis of Darwinian evolution. A countervailing view was the Mendelian school formulated chiefly by William Bateson. Bateson thought Mendel’s work indicated that more abrupt, large-scale differences were the major mechanism by which evolution progressed.
This conundrum was finally resolved in 1918 by the remarkable geneticist-statistician, Sir Ronald Fisher. Fisher’s paper The Correlation Between Relatives on the Supposition of Mendelian Inheritance invoked a continuous variation model that could derive from diverse discrete loci. Other central figures who, from this starting point, helped propound the modern synthesis include Theodosius Dobzhansky, J. B. S. Haldane, Sewall Wright, Julian Huxley, George Gaylord Simpson, and Ernst Mayr. Clearly, the signal achievement of the modern synthesis is how it not only joined Mendelian and Darwinian perspectives on evolution but also placed the synthesis within a robust mathematical framework.
Among Dobzhansky’s many contributions, he operationally defined evolution as being “a change in the frequency of an allele in a gene pool” and also famously averred that “nothing in biology makes sense except in light of evolution.” For his part, Haldane firmly reasserted the primacy of natural selection as mathematically compatible with Mendelian genetics. Wright identified the importance of genetic drift in the course of evolution. Huxley, much like his grandfather T. H. Huxley, helped popularize evolution for a broader audience. Simpson, given his expertise as a paleontologist, introduced important considerations that enriched and leavened sometimes weighted mathematical reductionism of the modern synthesis. Mayr, a superb field biologist and trained physician, contributed in many ways, but perhaps particularly with his naturalist’s understanding of the importance of mating schemes and population isolates.
Other imminent population geneticists of the mid-to late 20th century include Motoo Kimura, Richard Lewontin, Luigi Cavalli-Sforza, John Maynard-Smith, William Hamilton, and George Williams. Kimura, with innovative diffusion equations, calculated probability and time to fixation of alleles, whether adaptive, deleterious, or the heretofore underappreciated but very important neutral trait. Lewontin was a pioneer in molecular studies of evolution. Cavalli-Sforza, a research physician among the first to study how modern genomes constitute an “archeological” record of inheritance and prehistory, greatly systematized the genetical study of demography in a manner free of the racialist bias that constrained earlier eugenics. Maynard-Smith educed remarkable genetic and evolutionary psychological insights as he infused population genetics with game-theoretical mathematics. Hamilton published in zoology and genetics, including his theoretical analyses that established a rigorous genetic basis for the existence of kin selection, now among the most widely cited papers in all of science. Williams promoted a genocentric view that has taken Darwinian ideas in new directions.
Beyond the modern synthesis, more recent advances in biochemical techniques have fostered the study of single genes (or other markers) at the molecular level. Whatever may be the methods used to identify genes and their alleles, either qualitatively or quantitatively, population genetics relies on statistical analyses of allele frequencies to understand and make predictions about gene flow in populations past, present, and future.
- Dobzhansky, T. (1937). Genetics and the origin of species. New York: Columbia University Press.
- Fisher, R. A. (1930). The genetical theory of natural selection. Oxford: Clarendon Press.
- Haldane, J. B. S. (1932). The causes of evolution. London: Longman, Green.
- Maynard-Smith, J. (1982). Evolution and the theory of games. Cambridge: Cambridge University Press.
- Mayr, E. (1942). Systematics and the origin of species. New York: Columbia University Press.