Divergent Responses to Selection by Two Populations of Drosophila melanogaster

Abstract

Precise measurements of the time between fertilization and hatching of eggs show that different populations have different means. Slight temperature differences produce substantial differences in mean hatching time, but between any two populations the difference between the means is more constant than the means themselves. In 3 periods of sampling over a lapsed time of 8 months 2 large populations gave a mean difference in hatching time of from 6 to 7 minutes in a total of over 1,100. In all 3 periods this difference was significant at the 95% level. Between the first and second of these periods the populations were subjected to 24 days of higher temperature. In another period of sampling beginning 4 days after the higher temperature ceased, the difference in means was at first reversed and during the next 6 weeks, gradually returned to its former value where it remained for 3 months. Evidence is given that these changes were genetic, resulting from selection. The meaning of these observations is discussed in terms of genetic, developmental and population systems and in relation to mechanisms of evolution.