Population genetics is the branch of genetics that studies the hereditary make-up of populations. It measures the frequencies of genes and genotypes in a population and explains how they stay constant or change over time under mutation, natural selection, genetic drift, gene flow and non-random mating. Its unit of study is the Mendelian population, a group of interbreeding individuals sharing a common gene pool. It is the theoretical core of the study of human evolution and variation.
This note covers the meaning, basic concepts, history, scope and anthropological relevance of population genetics for the UPSC Anthropology Optional. It falls under Paper I, topic 9.3 (genetic polymorphism and selection, Mendelian population, Hardy-Weinberg law, and the factors that change gene frequencies) of the Anthropology Optional syllabus.
Key points at a glance
- Definition: the study of gene and genotype frequencies in populations and of the forces that change them.
- Unit: the Mendelian population (Dobzhansky, 1950).
- Baseline: the Hardy-Weinberg law (1908), which states when frequencies stay constant.
- Forces of change: mutation, selection, genetic drift, gene flow and non-random mating.
- Founders: R. A. Fisher, J. B. S. Haldane and Sewall Wright.
- Outcome: the modern synthetic theory of evolution, joining Darwin's selection with Mendel's genetics.
- In anthropology: human origins and migrations, human variation, adaptation, and the genetics of disease.
Meaning and definition
Classical genetics asks how a trait passes from parents to offspring in a family. Population genetics asks a different question: how common is a gene in a whole population, and why does that change? Because evolution is, at bottom, a change in the genetic composition of populations over generations, population genetics gives the deepest account of how evolutionary change occurs.
| Basis | Mendelian (classical) genetics | Population genetics |
|---|---|---|
| Unit of study | The individual and the family | The population |
| Question | How is a trait inherited? | How frequent is a gene, and why does its frequency change? |
| Method | Crosses and pedigrees | Sampling, statistics and mathematical models |
| Result | Ratios among offspring | Allele and genotype frequencies |
| Bearing on evolution | Explains heredity | Explains evolutionary change |
Basic concepts
| Concept | Meaning |
|---|---|
| Mendelian population | A group of sexually reproducing, interbreeding individuals that share a common gene pool (Dobzhansky). The smallest local unit is called a deme. |
| Gene pool | The total of all genes, in all their forms, carried by the members of a population. |
| Allele | One of the alternative forms of a gene at a locus, such as A and B in the ABO blood group system. |
| Allele (gene) frequency | The proportion of a given allele among all the alleles at that locus in the population. |
| Genotype frequency | The proportion of individuals with a given genotype. |
| Genetic polymorphism | The occurrence of two or more alleles at a locus, each at a frequency too high to be maintained by mutation alone, conventionally 1 per cent or more. |
| Fitness | The relative reproductive success of a genotype. |
| Effective population size | The number of individuals who actually contribute genes to the next generation; usually smaller than the census number. |
The central principle: the Hardy-Weinberg law
In 1908 the English mathematician G. H. Hardy and the German physician Wilhelm Weinberg showed independently that, in a large population mating at random, allele and genotype frequencies remain constant from generation to generation, provided no evolutionary force acts on them. For two alleles with frequencies p and q, the genotype frequencies are p², 2pq and q².
The law describes a population that is not evolving. Its value lies in the reverse reading: where observed frequencies depart from those expected, some force must be at work.
The forces that change gene frequencies
| Force | What it does | Human example |
|---|---|---|
| Mutation | Creates new alleles; the ultimate source of all variation | The mutation producing haemoglobin S |
| Natural selection | Changes frequencies according to differences in fitness | Persistence of the sickle-cell allele in malarial regions through heterozygote advantage |
| Genetic drift | Random change, strongest in small populations; includes founder effect and bottleneck | Unusual allele frequencies in small isolated communities |
| Gene flow (migration) | Moves alleles between populations and makes them more alike | Admixture between neighbouring groups |
| Non-random mating | Inbreeding and assortative mating change genotype frequencies, raising homozygosity | Consanguineous marriage and endogamy |
Population genetics studies these forces singly and in combination, within and between populations.
A short history of population genetics
The field developed well outside anthropology. Its history is the story of how Darwin's theory and Mendel's laws, long thought incompatible, were brought together.
Timeline
| Period | Development |
|---|---|
| 1859 | Darwin's Origin of Species: natural selection, but no sound theory of heredity. |
| 1866; rediscovered 1900 | Mendel's laws of inheritance. |
| 1900 to 1918 | Dispute between the biometricians, who studied continuous variation, and the Mendelians, who studied discrete traits. |
| 1908 | The Hardy-Weinberg law. |
| 1918 to 1932 | Fisher, Haldane and Wright found theoretical population genetics. |
| 1937 to 1950 | The modern synthesis: Dobzhansky, Mayr, Huxley, Simpson, Rensch and Stebbins. |
| 1953 onwards | The structure of DNA; molecular methods; protein and then DNA polymorphisms. |
| 1968 | Motoo Kimura's neutral theory: much molecular variation is shaped by drift, not selection. |
| 1987 | Cann, Stoneking and Wilson use mitochondrial DNA to trace modern human origins to Africa. |
| 2001 onwards | The human genome sequence, large surveys of human variation, and ancient DNA. |
The three founders
| Basis | R. A. Fisher (1890–1962) | J. B. S. Haldane (1892–1964) | Sewall Wright (1889–1988) |
|---|---|---|---|
| Background | English statistician and geneticist | British-born physiologist and geneticist who settled in India in 1957 and became an Indian citizen | American geneticist |
| Key works | 1918 paper on the correlation between relatives; The Genetical Theory of Natural Selection (1930) | "A Mathematical Theory of Natural and Artificial Selection" (1924–1934); The Causes of Evolution (1932) | "Evolution in Mendelian Populations" (1931) |
| Main ideas | Continuous traits result from many Mendelian factors of small effect; fundamental theorem of natural selection; balanced polymorphism and heterozygote advantage; ideas on sexual selection and the sex ratio | Measured the rate at which selection changes gene frequencies; first estimates of human mutation rates and human linkage; interaction of mutation and migration with selection | Genetic drift (the "Sewall Wright effect"); inbreeding coefficient and F-statistics; shifting balance theory; adaptive landscape |
| Tools | Analysis of variance; maximum likelihood | Quantitative models of selection under inbreeding, incomplete dominance and migration | Path analysis |
| View of evolution | Selection acting on many genes of small effect in large populations | Strong selection on single genes; attention to migration and gene interaction | Drift, subdivision and migration matter alongside selection; gene interaction (epistasis) is important |
Their differences. Fisher and Haldane held that natural selection is the chief force shaping the genetic structure of populations. Wright argued that chance and migration also contribute. In his shifting balance theory, a large population divided into small, partly isolated groups evolves fastest: drift lets some groups reach new combinations of genes, selection favours the better adapted, and gene flow spreads the gain. The disagreement was productive, because the three shared the same mathematical approach.
The modern synthesis
The mathematical theory explained adaptive change within populations. It did not explain the origin of species and of biological diversity. Naturalists studying geographical variation knew the importance of isolation in forming new species, but the two groups worked apart.
- Theodosius Dobzhansky, in Genetics and the Origin of Species (1937), joined the theory to observations on natural populations.
- Ernst Mayr (speciation and the biological species concept), Julian Huxley (who gave the synthesis its name), G. G. Simpson (the fossil record), Bernhard Rensch and G. L. Stebbins (plants) completed it.
The result is the modern synthetic theory of evolution: evolution is change in the gene frequencies of populations, produced by mutation, recombination, selection, drift and gene flow, with isolation leading to new species. One of its lasting products was the concept of the Mendelian population. A debate that continued afterwards was whether selection acts on the gene, the individual or the group.
Scope of population genetics
| Area | What is studied |
|---|---|
| Description of variation | Documenting allele frequencies at polymorphic loci: blood groups, serum proteins, enzymes, HLA and DNA markers |
| Theory | Mathematical models predicting how frequencies behave under each force |
| Population structure | Subdivision, inbreeding, mating patterns and genetic distance between groups |
| Evolutionary history | Inferring past population size, migrations, admixture and time of divergence |
| Adaptation | Detecting the signature of selection in the genome |
| Medical genetics | Frequency and distribution of genetic disorders; susceptibility and resistance to disease |
| Pharmacogenomics | Genetic variation in response to drugs |
| Applied fields | Genetic counselling, forensic identification, conservation, and plant and animal breeding |
With the tools of molecular genetics and bioinformatics, variation can now be recorded across the whole genome in very large samples, which has greatly widened the scope of the subject.
Relevance of population genetics in anthropology
Population genetics is an integral part of biological anthropology.
1. Human origins and dispersal
Rebecca Cann, Mark Stoneking and Allan Wilson (1987), and Linda Vigilant and colleagues (1991), compared mitochondrial DNA from people around the world. They found the greatest diversity in Africa and traced all lineages to an African ancestor. This supported the "Out of Africa" or recent African origin model, in which modern humans arose in Africa and later spread across Asia, Europe and beyond. Studies of the Y chromosome and of whole genomes have since agreed. Ancient DNA has added that small amounts of ancestry from Neanderthals and Denisovans survive in living people. See our note on the human fossil record.
2. Human variation
Our species is spread over the whole earth, yet differences between its populations are small. Richard Lewontin (1972) showed that most human genetic variation, roughly 85 per cent, lies between individuals within any one population, and only a small part between the major geographical groups. This pattern points to a recent common origin and to continued gene flow. It is the genetic basis for the anthropological conclusion that "races" are not discrete biological units.
3. Adaptation
Population genetics identifies genes that selection has favoured in particular environments: resistance to malaria (haemoglobin S, thalassaemia, G6PD deficiency), the ability to digest milk in adulthood among pastoral peoples, adaptation to high altitude among Tibetans and Andeans, and skin pigmentation in relation to sunlight.
4. Health and disease
By linking evolutionary and medical questions, the field explains why certain disorders are common in certain populations, and supports the development of diagnostic tests and treatments. Understanding normal variation is essential for interpreting any link between a genetic variant and a disease.
5. Social structure and genes
Marriage rules shape gene pools. Endogamy, consanguineous marriage, clan exogamy and population size determine inbreeding and drift, which is where social and biological anthropology meet.
Population genetics and India
- Haldane in India. J. B. S. Haldane worked at the Indian Statistical Institute, Kolkata, and later at Bhubaneswar, and encouraged studies of Indian populations.
- Endogamy. India's thousands of endogamous castes and tribes are, in effect, separate Mendelian populations. Many show strong founder effects, with recessive disorders particular to a community.
- Ancestry. Genome studies indicate that most Indian populations are mixtures, in varying proportions, of a small number of ancestral groups, followed by a long period of endogamy.
- Disease. The sickle-cell trait is frequent among several tribal populations of central, western and southern India. Thalassaemia and G6PD deficiency also show marked regional and community patterns.
- Institutions. The Anthropological Survey of India and national genome variation projects have mapped genetic diversity across the country.
Using this topic in a UPSC answer
- Define population genetics and the Mendelian population in two lines, citing Dobzhansky.
- State the Hardy-Weinberg law as the baseline and list the five forces that disturb it.
- Give the three founders with one contribution each, and their difference over selection and drift.
- Explain the modern synthesis as the union of Darwin and Mendel.
- For relevance, use three examples: Out of Africa, Lewontin's finding on variation, and sickle cell and malaria.
- Add an Indian point: endogamy, founder effects and sickle cell among tribes.
For presentation, see our guide to anthropology answer writing with diagrams, thinkers and case studies.
Frequently asked questions
What is population genetics?
Population genetics is the branch of genetics that studies the frequencies of genes and genotypes in populations and the forces, such as mutation, selection, drift and gene flow, that keep them constant or change them over time.
What is a Mendelian population?
A Mendelian population, as defined by Dobzhansky, is a group of sexually reproducing, interbreeding individuals who share a common gene pool.
Who are the founders of population genetics?
R. A. Fisher, J. B. S. Haldane and Sewall Wright founded theoretical population genetics between about 1918 and 1932, by showing mathematically how natural selection works on Mendelian inheritance.
What is the scope of population genetics?
Its scope includes describing genetic variation, modelling evolutionary forces, analysing population structure, reconstructing population history, detecting adaptation, and applying this knowledge to medicine, forensics and conservation.
What is the Hardy-Weinberg law?
The Hardy-Weinberg law states that in a large, randomly mating population, allele and genotype frequencies remain constant from generation to generation in the absence of mutation, selection, migration and drift.
How did Fisher, Haldane and Wright differ?
Fisher and Haldane regarded natural selection as the main force of evolution. Wright gave greater weight to genetic drift, population subdivision, migration and the interaction between genes.
What is the modern synthetic theory of evolution?
It is the theory, formed between the 1930s and 1950s, that unites Darwin's natural selection with Mendelian genetics. It explains evolution as change in gene frequencies in populations, with isolation leading to new species.
Why is population genetics important in anthropology?
It allows anthropologists to trace human origins and migrations, measure and explain human variation, identify genetic adaptations to environment, and understand the distribution of genetic diseases among populations.