Because invasive species must colonize environments to which they are evolutionarily naïve, directional selection on exotic species may be intense. Corroborating this hypothesis, several recent studies have documented evolutionary change in exotic species. In contrast, directional selection might be expected to be weaker (and stabilizing selection stronger) for native populations that have had the opportunity to adapt to local environmental conditions. Here, we quantitatively test whether the pattern and strength of natural selection differs between invasive and native populations. Specifically, we constructed a database of published selection analyses to: 1) Compare the strength of natural selection on native vs. invasive species, 2) Compare the relative strength of genetic correlations that may constrain adaptive responses, and 3) Identify traits underlying adaptation to novel environments. We hypothesize that the rapid evolutionary changes observed during biological invasions are due to strong directional selection on invasive species and the breakdown of genetic correlations that commonly constrain evolutionary responses in the native range.
Results/Conclusions
Of the 434 estimates of selection coefficients from 59 published studies included in our analyses to date, directional selection was significantly stronger on exotic species than native species (χ2 = 6.14, P < 0.013). This strong directional selection on invasive species should lead to adaptive evolutionary change, assuming that the traits under selection are genetically variable. In contrast to the patterns observed for directional selection coefficients, natives tended to have more negative quadratic selection coefficients, although this difference was not statistically significant (χ2 = 0.97, P = 0.32). Because stabilizing selection causes negative quadratic selection coefficients, this result is consistent with native taxa being better adapted to their local environmental conditions. Our findings suggest that exotic species face novel environmental conditions that exert strong selection and are consistent with examples of contemporary evolutionary change in invasive species. Thus, our results highlight the relevance of evolutionary processes to biological invasions.