GENETIC STRUCTURE OF POPULATIONS. II. VIABILITIES AND VARIANCES OF HETEROZYGOTES IN CONSTANT AND FLUCTUATING ENVIRONMENTS

Abstract

Changes in frequencies of (lethal + semilethal) chromosomes can be correlated with changes in environmental variables (Band and Ives, 1961). This suggests that these constituents of the genetic load are maintained in the population in dynamic equilibrium with the natural environment. It also implies that the fitness of heterozygotes carrying drastic/ drastic (d/d), drastic/nondrastic (d/nd), and nondrastic/nondrastic (nd/nd) combinations is neither uniform in the same environment nor constant in different environments. Band and Ives (1963) have shown that the significantly negative correlation between the frequency of drastics and temperature range: (1) can be used to predict an increase or decrease in the frequency of drastics in chromosome samples; (2) indicates that heterozygotes in the same environment tend to differ in viability and to contribute to the genetic homeostasis of the population in proportion to their relative frequencies and genotypic fitness values; (3) suggests that such variants do constitute mainly a balanced load in the population. Because of the concept of a dynamic relationship between environment and genetic structure, certain environments are expected to affect the fitness of heterozygous carriers of drastic chromosomes adversely; other environments to favor them above the carriers of only nondrastic chromosomes. Notably, environments with wide temperature ranges between the daily maxima and minima should be unfavorable for heterozygotes carrying drastic chromosomes; those with narrow temperature ranges should favor such heterozygotes. The positive relationship between mean temperature, as a measure of warmth or coolness in the environment, and the d frequency maintained after 1950 (Band and Ives, 1961) indicates, however, that the selection for or against drastic chromosomes is not a simple process. To test the validity of the above working hypothesis, the same 100 chromosome combinations have been tested in three environments: a constant temperature of 770 F (250 C), a narrow range with 770 F mean, and a wide range also with 770 F mean. As there is the possibility that the environment can influence the gametic contribution to the next generation (Ives, 1945; Novitski and Hanks, 1961), chromosome rederivations were undertaken for wild-type males of le/non-lethal constitution produced in 20 crosses in each of the environments tested. Chromosomes used in this study were second chromosomes from the 1960 collection of Drosophila melanogaster at South Amherst, Mass.