Selective Mating in Drosophila Melanogaster.

LTHOUGH DARWIN (1871) believed that sexual selection was quite dis-A tinct from natural selection, the modern definitions of natural selection (HALDANE 1932; HUXLEY 1938, 1942) include the mating process among the forces of natural selection. DARWIN'S emphasis on the importance of male competition and the preferences of the females in sexual selection has been shown to have been misplaced in most cases. However, it is realized that the true measure of success by the individual, in an evolutionary sense, is not merely survival, but the number of offspring produced. Therefore, any understanding of the nature of the genetic changes occurring within a population requires some knowledge of the sexual behavior and the mating system. Since such diverse factors as mutation pressure, migration, and isolation, as well as selection, can cause evolutionary change, a measuring device applicable to all was desirable. The conditions under which these and other factors are most effective in causing genetic changes, and the results of each acting separately or in combination have been predicted theoretically by the statistical treatment of gene frequencies in populations (FISHER 1930; HALDANE 1932 ; WRIGHT 1931, 1940a). The calculations have been greatly simplified by the assumption of random mating within a population. Even in studies of the breeding structure itself (WRIGHT 1940b, 1943, 1946), it has been assumed that, where the opportunities for mating are equal, mating is at random. The HARDY-WEINBERG formula shows that in a sexually reproducing, random breeding population where the component genotypes are equally successful in surviving, the relative frequencies of various genes in the population remain constant. Hence, the postulation of a random breeding population permits the study of the effects of different forces on a system in equilibrium. The present study was designed to ascertain whether or not mating actually was at random between the wild type and four mutant types of Drosophila melanogaster. If mating was found to be non-random, it was desired to determine the mating system. The results of these experiments could then serve to indicate how much of the change in gene frequency in a population is due to the effects of selective mating. Using the same mutations and their wild alleles in " population bottles'' (REED and REED 1948), LUDWIN (1948) measured the variations in frequency of the genes during several generations of competition between them. A comparison of his r,esults with the results of these experiments should indicate the …