Population genetics seeks to understand the laws that govern and forces that influence the amount of genetic variation within populations and changes in genetic variation over time. The concept of the gene pool provides a model for thinking about the transmission of genetic variation from one generation to the next for an entire population. Basic population genetic theory starts with an idealized population that is infinite in size and in which mating is random. In such a population, the Hardy–Weinberg law defines the relationship between allele frequencies in the gene pool and genotype frequencies in the population.

Real populations usually deviate to a small or large degree from the Hardy–Weinberg model. One source of deviation comes in the form of nonrandom or assortative mating. If individuals preferentially mate with others who share a similar phenotype, then there will be an excess of homozygotes at genes controlling that phenotype compared to Hardy–Weinberg expectations. When individuals mate more frequently with relatives than expected by chance, then there will be an excess of homozygous genotypes throughout the entire genome and the population becomes inbred. Even when local populations of a species conform to Hardy–Weinberg expectations, those populations are apt to be isolated from other populations at distant locations. Thus, a species often consists of a series of genetically distinct subpopulations; that is, species show population genetic structure.

Several forces can add new variation to a population or remove existing variation from it. Mutation is the ultimate source of all genetic variation. Population geneticists have determined reasonably precise estimates of the rate at which new mutations arise in populations. Migration can also bring new variation into a population. Migration results in some individuals who are genetically admixed, having ancestry from multiple populations. Genetic recombination can also add variation to populations by recombining alleles into new haplotypes.

Two forces control the fate of genetic variation in populations. First, genetic drift is a random force that can lead to the loss or fixation of an allele as a result of sampling error in finite populations. Drift is a strong force in small populations and a weak force in large ones. Second, natural selection drives changes in allele frequencies in populations over time. Alleles that enhance the fitness of the individuals that carry them will rise in frequency and can become fixed, while deleterious alleles that reduce fitness will be purged from the population.

The fundamental goal of population genetics is to understand the relative contributions made by mating systems, mutation, migration, recombination, drift, and natural selection to the amount and distribution of genetic variation in populations. In this chapter, we have seen how research in population genetics has both developed the basic theory and collected a vast amount of data to achieve this goal. Our understanding of the population genetics of our own species is remarkably detailed.

Finally, the methods and results of population genetics both inform us about the evolutionary process and have practical applications to issues facing modern societies. Population genetic theory and analyses play important roles in the management of endangered species, the identification of perpetrators of crimes, plant and animal breeding, and assessing the risks that a couple will have a child with a disease condition.

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