The study of the distribution of inherited variation among a group of individuals of the same species, and of how their genetic makeup changes over time—the process of evolution. The potential for change depends on the sum total of alleles that are available to the organisms (the gene pool), which is determined by, e.g. the extent of genetic polymorphism, mating patterns, migration between different populations, the founder effect, and the rate of new mutations. The direction of genetic change depends primarily on the selection pressures and the fitness of reproducing individuals as determined by their genotypes. In many cases alleles have little or no effect on fitness, and their frequencies may change in a random way (see genetic drift). The idealized formulation of genotype frequency, given no selection, random mating, infinite population size, and no migration, is predicted by the Hardy–Weinberg equilibrium.