INTERACTIONS BETWEEN DENSITY-DEPENDENT AND AGE-SPECIFIC SELECTION IN DROSOPHILA-MELANOGASTER

1. Density-dependent natural selection and age-specific natural select ion are important determinants of life-history evolution. A variety of laboratory populations of Drosophila melanogaster have been created t o study the effects of these selection mechanisms. 2. Two types of pop ulations have been selected for reproduction early (B) and late (O) in life. These have exhibited changes in life span and resistance to str esses, such as desiccation, starvation, ethanol vapours and flying to exhaustion. 3. Similarly, two types of populations have been selected at high adult and larval densities (K) and low adult and larval densit ies (r). These have exhibited changes in characters like larval feedin g rates, pupation height and minimum food required for successful pupa tion. 4. To study whether age-specific and density-dependent selection act on the same traits either directly or through indirect effects, s uch as pleiotropy or linkage, we have examined the B and O populations for the traits that have become differentiated in the r and K populat ions and vice versa. 5. In general, there is a lack of similar respons e, except for starvation resistance which is greater in the K populati ons than the r populations. 6. We have tested, for the first time, lon gevity in all four types of populations as a function of adult density . The O populations show greater longevity than the B populations at a ll densities and this difference does not depend on density. In contra st, the K populations are able to resist the decline in longevity caus ed by increasing density much more effectively than are the r populati ons. 7. Lastly, a new set of populations, called CU, has been derived from the B populations and is maintained by crowding the larval life s tage but raising adults under low densities. The CU populations have e volved increased feeding rates, pupation height and larval viability a t high density relative to the B populations. These changes parallel t he changes seen in the r and K populations and demonstrate the importa nce of crowding in the larval stages for much of the evolution seen in the r and K populations.