Population genetics is a genetic branch to study genetic variation among individuals within groups, and the genetic basis for evolutionary change, seeking to understand how patterns vary geographically and through time. The genetic structure of a population can be described by the genotype frequency and/or allele frequency.
Assume the population is infinitely large and mating is random, there is no natural selection, no mutation and no migration, the allele frequency of the population remain constant: p2 + 2pq + q2 = 1. This is the Hardy-Weinberg equation, which can be used in many circumstances, for example, to estimate allele frequency of certain diseases.
Genetic variation in space and time
Natural populations have genetic variance within themselves, which can be measured by a number of methods in a laboratory. FIS is used to determine the heterozygosity of a population, while FST is used to examine whether two populations are isolated or not. These are called Sewall Wright’s fixation indices.
Forces that change gene frequencies
Most populations do not meet Hardy-Weinberg equilibrium conditions, allele frequencies change, and the population’s gene pool evolves. There are 4 major forces that change the gene frequencies: mutation, genetic drift, immigration and natural selection. Mutation has low rate, creates small changes, increases genetic variation and is balanced with natural selection and drift. Genetic drift is random allele change by chance, it decreases variation due to loss of alleles, produces divergence and substantial changes in small populations. Migration increases effective population size and decreases divergence by encouraging gene flow (and reduces drift). It creates major changes in allele frequencies. Natural selection increases or decreases genetic variation depending on the environment, increases or decreases divergence. It continues to act after equilibrium has been achieved, and is balances with other forces, e.g., mutation-selection balance.