Density-dependent natural selection does not increase efficiency

Summary Populations ofDrosophila melanogaster kept at high population density (K-selected) for 125 generations have higher larval viability than populations kept at low densities (r-selected) when both are raised under crowded conditions. In additionK-selected adults that emerge from crowded cultures are larger than theirr-selected counterparts. These differences cannot be explained by differences in efficiency of food use. The minimum food required for successful pupation is actually greater in theK-selected populations. I conjecture that there may be a trade-off between minimum food requirements and competitive ability, which has changed substantially in theK-selected populations. The possibility thatK-selected larvae can dig more more deeply and gain access to unused food is examined and rejected as a possible explanation of the viability differences. Evidence is provided supporting the hypothesis that the differences in viability may be due to an increased tendency of theK-selected larvae to pupate off the surface of the medium.