A GENETIC-POLYMORPHISM MAINTAINED BY NATURAL-SELECTION IN A TEMPORALLY VARYING ENVIRONMENT

Environments that are crowded with larvae of the fruit fly, Drosophila melanogaster, exhibit a temporal deterioration in quality as waste pr oducts accumulate and food is depleted. We show that natural selection in these environments can maintain a genetic polymorphism with one gr oup of genotypes specializing on the early part of the environment and a second group specializing on the late part. These specializations i nvolve trade-offs in fitness components. The early types emerge first ii-om crowded cultures and have high larval feeding rates, which are p ositively correlated with competitive ability but exhibit lower absolu te viability than the late phenotype, especially in food contaminated with the nitrogenous waste product, ammonia. The late emerging types h ave reduced feeding rates but higher absolute survival under condition s of severe crowding and high levels of ammonia. Organisms that experi ence temporal variation within a single generation are not uncommon, a nd this model system provides some of the first insights into the evol utionary forces at work in these environments.