GENETICS OF NATURAL POPULATIONS. XXI. CONCEALED VARIABILITY IN TWO SYMPATRIC SPECIES OF DROSOPHILA

Abstract

ATCRAL populations of several species of Drosophila examined in this N respect carry great stores of concealed genetic variability. This variability may be brought to light, and its quality and quantity may be measured, by means of fairly simple genetic techniques. Individuals are obtained which are homozygous for, that is, carry in duplicate, certain chromosomes derived from known progenitors collected in the natural habitats of the species. Such homo-zygotes are often deficient in viability, or sterile, or show various structural or physiological abnormalities which distinguish them from " normal " or " wild " flies. Concealed genetic variability exists probably in all sexually reproducing and cross-fertilizing organisms including man. Its biological function is, however, little understood. I t may be important as a store of genetic raw materials from which new adaptive genotypes are built in the process of evolution. The concealed variability is also important as a source of hybrid vigor, or heterosis. On the other hand, it is a source of the poorly adapted variants and hereditary diseases which lower the immediate fitness of some members of natural populations. Little is known about the agencies which determine the quality and quantity of the store of concealed variability in a given species or population. Comparison of these stores in closely related species which differ in their ecology and reproductive biology is a promising method of investigation of the problem at hand. The present article reports such a comparison for two species, Drosophila pseudoobscura and Drosophila persimilis. These forms are sibling species, very similar in external morphology, and with similar chromosomes which carry presumably the same gene loci, although arranged in somewhat different linear orders. The two species occur together in some localities in the western United States, making it possible to collect samples of populations living in the same general environment.