Minor Climatic Shifts and Genetic Changes in a Natural Populations of Drosophila melanogaster

Abstract

Genetic changes in the South Amherst. Massachusetts, Drosophila melanogaster natural population have been found to have occurred at times of minor but significant climatic shifts in the area. These involve rainfall and average daily temperature range, May-October; the two variables appear inversely related. The time period spans the breeding season for the species in Amherst. Lethal and semilethal frequencies among samples of second chromosomes from the population were known to be negatively correlated with average daily temperature range of the week prior to collection; the frequencies in the first fall collections are positively associated with summer rainfall. Heterozygous classes have been shown to be differentially responsive to wide and narrow ranges between daily minimax temperatures, as predicted. Multiple prolonged shifts in both climatic variables were observed in the 1960s; the period 1961/62-1965/66 was the most severe of the past 40 years. Population response to these climatic shifts suggests a genetic structure dependent upon an interplay between stabilizing and disruptive selection, both acting at the heterozygote level. Developmental homeostasis appears to be maintained by stabilizing selection, while lethal and semilethal frequencies seem to be maintained by disruptive selection. Both appear subject to directional selection also acting at the heterozygote level. An adaptive role for lethals and semilethals is proposed, and consequences for the population are discussed. Similar climatic changes in rainfall and temperature range can also be identified in data from Lansing, Michigan.