Genic Variation in a Natural Population

-The understanding of the speciation process can best be achieved by a knowledge of the genic differences between two closely related species and the correlation of such differences with the characteristics which demarcate the two species. D. persimilis and D. pseudoobscura are two sibling species differing from each other in morphology, ecological preferences, and behavior, and no hybrids exist in nature. I have compared the genetic changes in 24 loci of 25 strains of D. persimilis from Mather, California, with those in D. pseudoobscura by the method of acrylamide gel electrophoresis. The proportion of the genome heterozygous in individual D. persimilis is 10.5 per cent, which is comparable to the heterogeneity found in similar D. pseudoobseura populations. I did not find any locus which is monomorphic or polymorphic for entirely different allele(s) than in D. pseudoobseura. The different frequencies of shared alleles in the two species can only be explained by selection acting differentially in these species, since the frequencies of different alleles at various loci in several populations of D. pseudoobseura are very similar or show a very stable pattern of association with the third chromosome gene arrangements. The process of speciation can best be understood by the study of the kind and magnitude of genetic changes in the genome of two closely related species. Since a great deal of genetic heterogeneity exists in a natural population of a species,1 2 these goals can only be achieved by examination of frequencies of various alleles at a large number of randomly chosen loci in several strains of the two species taken from various populations. Such a study would provide an over-all picture of the genetic differences that have occurred in the formation of the two closely related species. The best available information on the general genetic differentiation between species is the work of Hubby and Throckmorton3' 4on the differences in the proteins between single strains of various Drosophila species. Other studies have included the examination of changes in chromosome number and differences in gene arrangements in various Drosophila species and a comparison of the primary sequences of proteins such as insulin, adrenocorticotropin, melanotropin, vasopressin, oxytocin, hypertensin and hemoglobin,' and cytochrome c.6 While the study of changes in chromosome number and differences in gene arrangements in various species does not tell us anything about evolutionary diversification of a locus, the primary sequence studies do not take into consideration the extent of polymorphism within a species. Ideally, what one needs to do is to determine the amino acid sequence of a large number of enzymes and proteins in single individuals of many strains of a species. However, this seems untenable at the moment because of technical difficulties.