GENETIC VARIABILITY IN KOREAN POPULATIONS OFDROSOPHILA MELANOGASTER

Abstract

Concealed genetic variability has been found by many workers in various species of Drosophila. Much of this work is reviewed in Spencer (1947). Notable among other studies are those of Chino (1936a, 1936b and 1937) on Japanese populations of D. virilis and of Stone and his group (1957) on irradiated Pacific island populations of D. anannasae. In general it is clear that natural populations of many Drosophila species carry large amounts of heterozygus genetic variability, with marked quantitative and qualitative differences, however, between both intraand inter-specific populations. In addition, rates of change in genetic variability within populations have sometimes been reported as rapid (Dubinin, 1946), or slow (Ives, 1945). A primary step in speciation and organic evolution is the occurrence, dissemination and frequency distribution of mutations in natural populations. The quality and quantity of mutations which accumulates in space and time within a population or species plays a significant role in the evolutionary changes occurring in the species. Little is actually known about the mechanisms which control these factors in natural populations, although some progress has been made in elucidating concepts bearing on these mechanisms. Of particular use in this respect have been lethal mutations, whose detection is relatively simple and easy. The frequency of such mutants in a population is related at least theoretically to the breeding structure of the population and to the nature of its environment. If there are no heterozygous effects, the concentration of recessive lethals at equilibrium should be lower in small populations than in continuously large populations (Wright, Dobzhansky and Hovanitz, 1942; Dubinin, 1946). Further, their frequency should be lower in small populations, at comparable generations, even before equilibrium is reached (Prout, 1954). Lessening of inbreeding at any time in a population leads to an increase in frequency of recessive lethal, or partially lethal, genes in the population's gene pool. Hence the frequency of these mutations found in a sample of flies from a population is in a sense a yardstick for estimating comparative sizes of populations (Ives, 1945) excepting as a selection operates appreciably for (Mazing, quoted in Dubinin, 1946) or against (Wright et al., 1942; Stern et al., 1952; Cordeiro, 1952; Prout, 1952) such heterozygous mutations. There is increasing evidence of geographical differences in concentration of lethals and other genetic variability. Dubinin (1946) also reported periodical fluctuations in lethal frequency correlated with seasonal oscillations in population size, a phenomenon not confirmed by others (Wright et al., 1942; Ives, 1954; Goldschmidt et al., 1955). Further work is needed along this line. In a large breeding population gene frequencies are determined by counteracting, systematic evolutionary forcesmutation, selection and migration. In a small, island-type population, even in one subject to oscillations in climatic and ecological conditions, random changes in gene frequencies are also important, for they will sometimes by themselves cause fixation or elimination of certain alleles without respect to their selective value