Evolution and insect invasions

The role of evolution in biological invasions has been debated for decades, with discussion revolving around the importance of genetic variation, phenotypic plasticity, and the presence/absence of predators and competitors, among other factors. New genetic theory shows that genetic variation may not be limiting following genetic bottlenecks associated with colonization. And, recent work has documented that evolution can occur in invasive species and organisms that interact with invasive species. Together, these factors have been used to explain general observations associated with invasions, including long lag times and variation in rates of spread. However, many species, including some important insect pests, do not fit these general patterns--species can be successful invaders following successive genetic bottlenecks with little evidence of fitness costs at small population sizes. Further, the time periods typically cited for documentation of evolution associated with invasions are in the order of 100s of generations, not the few generations over which many invasions take place. Here, we use genetic studies of invasive insects to evaluate these conflicting observations and to try to explain the outliers. We focus on demographic features of insect invasions, including large dispersal potential, sizable population sizes, and rapid population growth, and evaluate the extent to which recent population genetic computation methods can provide insight into these parameters. We examine also differences in traits between invasive and resident species that facilitate invasion and ecological impacts. The work emphasizes that invasions can inform also the role of evolution in natural colonization over varying time periods.