Ancestral ecological regime shapes reaction to food limitation in the Least Killifish,Heterandria formosa

In populations with contrasting densities of conspecifics, we often see genetically-based differences in life histories. The divergent life histories could be driven by several distinct agents of selection, including, amongst other factors, variation in per-capita food levels, the intensity of crowding-induced stress, rates of pathogen transmission, mate encounter rates, and the rates with which waste products accumulate. Understanding which selective agents act in a particular population is important as the type of agents can affect both population dynamics and evolutionary responses to density-dependent selection. Here we used a full-factorial laboratory experiment to examine whether two populations of a small live-bearing freshwater fish, characterised by high-density/low-predation or low-density/high-predation conditions, are adapted to different per-capita food levels. As expected, fish from the higher density regime handled food limitation better than those from the lower density regime. Although the lower food level resulted in slower growth, smaller body size, delayed maturation and reduced survival in both populations, especially survival to maturity showed a highly significant population x food-level interaction. At low food, 75% of fish from the low-density population died, compared to only 15% of fish from the high-density population. This difference was much smaller at high food (15% vs. 0% mortality), and was mediated, at least partly, through a larger size at birth of fish from the high-density regime. While we cannot preclude other agents of selection from operating differently in the study populations, we demonstrate that selection at higher density confers a greater ability to cope with low per-capita food availability.