Does population stability evolve?

Abstract

Population stability ultimately depends on the life-history characteristics of individuals; thus, it may be indirectly affected by natural selection acting on various life-history traits. This study investigates the efficacy of natural selection in molding the stability of populations living at an unstable equilibrium. The stability of laboratory populations of Drosophila is affected by the relative amount of food given to larvae and adults. Environments with high larval food levels and low adult food levels (HL environments) tend to have asymptotically stable carrying capacities. Environments with low larval food levels and high adult food levels (LH environments) tend to exhibit unstable dynamics, like population cycles. In this experiment, 20 populations were created from two different types of source populations. Five of the source populations had evolved for 71 generations under crowded larval conditions and uncrowded adult conditions (CU populations), while the other five source populations had evolved for a comparable time in uncrowded larval and uncrowded adult conditions (UU). In this study, five replicate CU and UU populations each were placed in both the HL and LH environments, and total adult population counts and adult biomass were recorded for 45 generations. Every five generations, we also estimated the density-dependent fecundity function in each population, since population stability depends critically on the shape of this function. While we could document phenotypic evolution in these populations for several characters due to density-dependent natural selection, there was no detectable change in the population stability characteristics of the unstable LH populations. This result is consistent with either no evolution of population stability, or very slow change. Thus, while evolution in these populations affects important life-history characteristics, these changes appear to have no detectable effects on population stability.