Influence of Generation Time on the Rate of Response to Selection

We examined the influence of generation time on the rate of evolution of a trait under intense natural selection: pesticide resistance in arthropod pests. Previous empirical and theoretical analyses supported a positive linear relationship between the number of generations per year and the rate of evolution of pesticide resistance. To test this relationship, we assembled a data base that integrated information on resistance evolution, generation time, and other biological parameters for 682 North American arthropod pests. The data did not support a linear relationship between generations per year and the evolution of resistance, revealing instead a nonlinear and highly variable relationship, with peak rates of resistance evolution for species with intermediate generation times. This result was independent of the difference between introduced and native species and of differences among the major arthropod taxonomic orders in ability to evolve resistance. A reevaluation of evidence from analytical and computer-simulation models of resistance evolution suggests that the linear relationship between generations per year and resistance evolution is also without foundation in theory. An extension of a simple analytical model of resistance evolution suggests instead that the rate of resistance evolution is independent of generation time. We also find little support for the suggestion that species with many generations per year are regularly subject to elevated levels of selection for pesticide resistance. Per-generation fitness values for genotypes conferring resistance to pesticides or genotypes conferring increased fitness in response to any density-independent selective agent are related exponentially to generation time, resulting in the independence of generation time and the rate of response to selection in the simplest-case model. Generation time can influence the rate of resistance evolution; however, rather than acting in a simple, uniform manner, generation time interacts with a variety of genetic, ecological, and operational factors to produce a multitude of effects.