Although gene flow occurs in most populations, it is usually slight enough to be negligible for causing major shifts in allelic frequencies.
Finally, all the vast numbers of factors involved in reproduction must occur totally at random, including such factors as selection of a mate, fertility, percentage of zygotes that survive to birth, and the percentage of young that survive to reproduce. In fact, the Hardy-Weinberg Principle demands that each of the factors associated with reproduction must be independent of genotypic influence. Since most aspects of reproduction are inextricably influenced by genotype, non-random reproduction must be considered the norm. Non-random, genotype-influenced reproduction causes natural selection to occur. Natural selection is a process in which individuals with genotypic traits of adaptive superiority are more likely to survive to reproduce while individuals that lack genotypic traits of adaptive superiority are less likely to survive to reproduce. Thus, natural selection is said to exert selection pressure on all populations in favor of individuals with the genotypic traits of adaptive superiority.
Because it is impossible for the gene pool of any population to meet all four conditions of stability as mandated by the Hardy-Weinberg Principle (especially with regard to the requirements of having no mutations and random reproduction), it can be said that the gene pool of a population can never be in complete equilibrium and, thus, that evolutionary change, based largely on natural selection and to a lesser extent on genetic drift, mutation pressure, and gene flow, must be a common part of the
