The study of both experimental and theoretical consequences of mendelian heredity on the population level, in contradistinction to classical genetics which deals with the offspring of specified parents on the familial level. The genetics of populations studies the frequencies of genes, genotypes, and phenotypes, and the mating systems. It also studies the forces that may alter the genetic composition of a population in time, such as recurrent mutation, migration, and intermixture between

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... selection resulting from genotypic differential fertility, and the random changes incurred by the sampling process in reproduction from generation to generation. This type of study contributes to an understanding of the elementary step in biological evolution. The principles of population genetics may be applied to plants and to other animals as well as humans. See also: MENDELISM . Mendelian populations A mendelian population is a group of individuals who interbreed among themselves according to a certain system of mating and form more or less a breeding community. These individuals share a common gene pool which is the total genic content of the group. A mendelian population is the unit of study in population genetics. The population may be very large or very small, and is to be distinguished from species or varieties, which may consist of numerous isolated or partially isolated mendelian populations. Mendelian population is a genetic rather than a taxonomic term. Mendelian populations differ from each other in their genic content or chromosomal organization, not necessarily in their taxonomic features. The term deme, originally defined as an assemblage of taxonomically closely related individuals, has been used as a synonym for mendelian population. Gamodeme, a deme forming a more or less isolated local intrabreeding community, would be a better substitute. Mutation pr essur e Gene mutation arises from time to time in nature. The causes for mutation are not fully known, and thus it can be said that mutations arise "spontaneously." The effect of a new mutant gene is unpredictable and the gene is therefore said to mutate "at random." One property of mutation has been established: It is recurrent. Each type of gene mutates at a certain rate per generation. The rate is usually low-about 1 mutant in 10 , 5 -10 , 8 genes of a given sort, varying from locus to locus on the chromosomes, even under uniform conditions. Ionizing radiation, certain chemicals, heat, and some other agents increase the rate of mutation. See also: MUTATION . Let μ be the rate of mutation from an allele A to another form a per generation. If a fraction p of the genes of a Random drift The random drift of gene frequencies in finite populations is often called the Sewall Wright effect because of his analysis of its significance. The gene frequency of any generation is determined by the uniting gametes produced by the parents of the preceding generation. If the number of parents is limited and constitutes a random sample of the entire population, the gene frequency of the next generation will not remain exactly the same as that of the previous generation but will be subject to a random fluctuation on account of the sampling process. In a random mating population of N individuals, one-half of whom are males and one-half females, and maintaining the same population size, the variance of the gene frequency based on 2 N gametes is q (1 − q ) ∕ 2 N . The gene frequency may become a little higher or a little lower in the following generation. The smaller the population, the greater is the variance. This random process will continue to operate in all generations. In a sufficiently long time, the value of q will reach either the terminal value 0 or 1. Hence the random drift leads eventually to complete homozygosis for small populations. It can be shown that the limiting rate of reaching the state 0 or 1 is each 1 ∕ 4 per generation, so that the total rate of "decay" of genetic variability is 1 ∕ 2 N per generation. Naturalists have found numerous small isolated colonies (for example, snails in mountain valleys) with characteristics uncorrelated with the environmental conditions to substantiate the theory of random (nonadaptive) fixation. The effective size of a population is the actual number of individuals producing offspring and thereby responsible for the genetic constitution of the next generation. The random mating population with one-half males and one-half females, and producing the same number of offspring, is an idealized model. Any deviation from the ideal situation will have a different sampling variance and a different rate of decay. Equating these to the