Life-Historical Consequences of Density-Dependent Selection in the Pitcher-Plant Mosquito, Wyeomyia smithii

Abstract

Over its range, the pitcher-plant mosquito, Wyeomyia smithii, experiences a gradient in density-dependent development. In each of 12 populations from six geographical regions over a range of 19⚬ latitude and 900 m in altitude, we determined capacity for increase and its components at four densities. We reared experimental populations in intact pitcher plants receiving near-natural conditions of light, temperature, and feeding. Capacity for increase and its components, replacement rate and mean generation time, exhibited significant variation among densities and among localities within regions but not among regions. There was no trade-off among populations in capacity for increase at high and low densities. Consequently, variation in these fitness traits did not reflect major differences in climate or degree of densitydependent selection prevailing at the locality of origin. Hybrids between populations exhibited heterosis in capacity for increase and in both of its components. These results indicate that local populations of W. smithii can and have diverged with respect to capacity for increase and its components but that the simple effects of density alone are totally inadequate to explain the evolution of major demographic traits in this species. Since most organisms live in habitats that are more variable among localities than does W. smithii, we suspect that evolution at the level of the local population may be an important contributor to the evolutionary divergence of demographic traits in other species as well. Despite a lack of differences in density tolerance among zones, southern populations exhibited greater competitive ability than did northern populations. Competitive ability can therefore evolve independently of density tolerance and can be assessed only by direct competition of the genotypes, populations, or species concerned.