Population genetics is the branch of biology that provides deepest understanding of how evolutionary changes occur. Population genetics can broadly define as the branch of genetics concerned with the hereditary makeup of populations. It can also refers to the study of gene and genotype frequency and predicting the way these frequencies change or remain constant under the combined influence of various factors, over time, within and between populations. Population geneticists usually focus their attention on a Mendelian population, which is a group of interbreeding, sexually reproducing individuals that share a common gene pool (Dobzhansky, 1950).
Population genetics is relevant today as its theory along with technological inputs from the other branches such as molecular genetics and bioinformatics is contributing immensely to document the allele frequencies at polymorphic loci at a large scale and to infer past demography and the effect of evolutionary forces operation on human populations in effective ways. One such example of application of human population genetics to the study of human population genetics to the study of human evolution is support for “Out of Africa” hypothesis, suggesting that modern humans evolved in Africa and moved to Asia and Europe about 100,000 years ago (Cann et al., 1987; Vigilant et al., 1991).
Population genetics has also expanded its quest to understand the basis for genetic variation in susceptibility or resistance to disease and drug’s responses. In recent years, new technologies and methodologies have been developed specifically to explore the role of genetic variation in health and disease. Proper understanding of human genetic variation is essential for the correct interpretation of the link between genetic variation and disease thereby its identification can assist in the development of diagnostic tests as well as effective treatments.
A short history of population genetics
The initial development of the field of population genetics took place well outside the field of Anthropology. Although the ideas proposed by Darwin and Mendel had respective advocates, they remained incompatible for a very long time. The ideas were synthesized to explain the genetic transmission of natural selection for evolution mathematically by Ronald Fisher (1890-1962), J. B. S. Haldane (1892-1964), and Sewall Wright (1889-1988). Much of the core of population genetics can be traced back to these three men in the early parts of the twentieth century. Over time, their mathematical formulations were combined with observations from laboratory experiments, field studies, and the fossil record to develop what is often referred to as the synthetic theory of evolution, referring to the synthesis of information from a variety of biological and geological fields (Provine, 1971).
Ronald A. Fisher was an English statistician, evolutionary biologist, mathematician, geneticist and eugenicist. He developed some of the concepts that have perpetual relevance to the field of population genetics. To name a few, balanced polymorphism and heterozygote advantage, ANOVA and maximum likelihood (ML) estimation of linkage. He showed how correlation between relatives can be explained by cumulative effect of a large number of Mendelian factors each having a small effect. Subsequently he devised the analysis of variance (ANOVA) which enabled disproportionate of variance to elucidate the contribution of the factors. The other major contributions were the fundamental theory of natural selection, ideas on sexual selection, inclusive fitness and parental expenditure. This piece of work established that natural selection driven evolution is primarily a within species phenomenon.
Sewall Wright:-Wright was an American geneticist. He pioneered the now standard tools of population genetics, inbreeding coefficient and F statistics. He gave the concept of the Sewall Wright effect. He also propagated the interplay of genetic drift with other evolutionary forces for adaptation to occur. For this, he devised the method of path analysis and used path coefficients to describe the outcomes of inbreeding, Assortative mating and selection (Wright, 1921). In 1931 came his seminal paper about and titled Evolution in Mendelian Populations which established that undergoing a series of maladaptive changes precedes higher levels of adaptation. Genetic drift would result in small populations. In case of a larger population undergoing sub-division into smaller units of populations, some of the latter will be better adapted. Gene flow between the smaller populations can lead to a uniform distribution of these adaptations.
J. B. S. Haldane:- Haldane was a British naturalized Indian Scientist, Well known for his work in physiology, genetics and evolutionary biology. He wrote a series of papers on A Mathematical Theory of Natural and Artificial Selection (1924-1934) and the book The Causes of Evolution (1932). His research was important for his stress upon the quantification of the rates of change of a population’s characteristics and he introduced quantitative approaches for estimating human linkage maps using a maximum likelihood method and mutation rates. He assessed the interaction of mutation and migration with natural selection for the first time. He substantiated his studies on selection by considering inbreeding, overlapping generations, incomplete dominance, isolation, migration and fluctuations of intensity of selection (Provine, 1971). He laid more emphasis on strong selection of single genes, migration and epistasis in contrast to Fisher.
There were however academic differences between Fisher, Haldane and Wright but these differences only went on to improve the subject owing to the remarkable compatibility in their overall approach. The most conspicuous difference was the three’s opinions on natural selection being the most important factor in shaping of the genetic structure of a population. Fisher and Haldane, being ardent Darwinians, were pro while Wright believed that chance factors and migration did contribute to genetic variation. Wright also laid a lot more emphasis on epistatic interactions between genes than Fisher or Haldane.
In spite of having answers on adaptive changes in populations the questions on origin of biodiversity were still unsolved. Naturalists who were meanwhile studying geographic variations within species knew the role of geographical isolation in the origin of biodiversity and eventual formation of species. However the population geneticists were unaware of these findings. The findings on the origin of biodiversity by naturalists and population genetics of adaptive changes provided the basis for the “Evolutionary Synthesis” also called modern synthetic theory of evolution (Dobzhansky, 1937). One of the most profound outcomes of this synthesis was identification of Mendelian population that comprises individuals interbreeding among themselves and sharing a common gene pool, a spatiotemporal identity and undergoing evolutionary changes (Dobzhansky, 1950).
The modern synthetic theory was supported by works of Ernst Mayr, Julian Huxley and Bernhard Rensch (Meyer, 2005). The only conflicting idea in the synthesis was whether it was the gene or the individual that was undergoing selection. It was suggested that a gene can impart properties to the individual that would subsequently favor its selection (Sober, 1993).
Relevance of Human Population Genetics in Anthropology
The science of population genetics is an integral component of biological anthropology in understanding human evolution and in pursuing the goal of human origins vis-à-vis the divergence of human groups, Population genetics is the key to our understanding of human variation, and by linking medical and evolutionary themes; it enables us to understand the origins and impacts of our genomic differences. Despite current limitations in our knowledge of the locations, sizes and mutational origins of structural variants, the overall growth in this field has brought new insights into recent human adaptation, genome biology and disease association studies. Population genetics provides models for investigating the balance of evolutionary forces acting on genetic diversity. Studies that use these models have found that the evolution of contemporary human genetic diversity has occurred over the past several hundred thousand years or longer. Our species is geographically widespread, but shows low levels of differences among population groups suggesting persistent levels of gene flow as well as dispersal.