RAPID LABORATORY EVOLUTION OF ADULT LIFE-HISTORY TRAITS IN DROSOPHILA-MELANOGASTER IN RESPONSE TO TEMPERATURE

Three replicate lines of Drosophila melanogaster were cultured at each of two temperatures (16.5 degrees C and 25 degrees C) in population c ages for 4 yr. The lifespans of both sexes and the fecundity and ferti lity of the females were then measured at both experimental temperatur es. The characters showed evidence of adaptation; flies of both sexes from each selection regime showed higher longevity, and females showed higher fecundity and fertility, than flies from the other selection r egime when they were tested at the experimental temperature at which t hey had evolved. Calculation of intrinsic rates of increase under diff erent assumptions about the rate of population increase showed that th e difference between the lines from the two selection regimes became l ess the higher the rate of population increase, because the lines were more similar in early adulthood than they were later. Despite the inc reased adaptation of the low-temperature lines to the low temperature, like the high temperature lines they produced progeny at a higher rat e at the higher temperature. The lines may have independently evolved adaptations to their respective thermal regimes during the experiment, or there may have been a trade-off between adaptation to the two temp eratures, or mutation pressure may have lowered adaptation to the temp erature that the flies no longer encountered.