GENETIC STRUCTURE OF POPULATIONS. I. ON THE NATURE OF THE GENETIC LOAD IN THE SOUTH AMHERST POPULATION OF DROSOPHILA MELANOGASTER

Abstract

Background of the problem.-Populations of Drosophila, for the most part, consist of heterozygous individuals. When chromosomes are extracted and made homozygous, a wealth of genetic diversity is revealed. The effects of homozygosity for individual chromosomes range from complete lethality to supervitality, i.e., to a viability superior to the average heterozygote. Wallace and Madden (1953) and Dobzhansky and Spassky (1953) showed that in experimental populations of D. melanogaster and natural populations of D. pseudoobscura and D. persimilis, respectively, the average viability of the homozygotes was lower than that of heterozygotes, even when lethals and semilethals (to be referred to below as drastics or d) were excluded. The remaining array, called quasinormals or nondrastics (nd), also possessed greater genetic and environmental variances than the random heterozygotes. These results led to the conclusion (Dobzhansky and Wallace, 1953) that the heterozygotes were on the average more homeostatic than the homozygotes, and that this homeostasis was a consequence of a coadaptation by natural selection of the alleles or gene complexes found in the gene pools of the respective populations. This homeostasis of heterozygotes creates a population property which Lerner (1954, 1958) has called genetic homeostasis. Experiments on subvital or supervital quasinormal chromosomes have supported the above conclusion of coadaptation as a consequence of natural selection. Dob-