Abstract The amphipod, Gammarus lawrencianus Bousfield, was maintained for ≈ 26 generations in a laboratory environment that permitted constantly increasing population size. The intrinsic rate of population growth, r m , increased 1.7-fold as a result of heritable changes in survivorship and fertility. Yield of biomass of large animals (“harvestable” biomass) increased 2.6-fold for each 45-day harvest cycle. Yield is correlated with fitness in this environment and therefore increased, although not selected for directly. Changes in life history traits are interpreted in terms of selection for Darwinian fitness in a controlled environment. The potential for genetic domestication of Crustacea is proportional to the variance of the major components of fitness, which can be estimated experimentally.