| The house sparrow, Passer domesticus, is a small passerine bird species native to Eurasia and North Africa. Following a series of human-mediated introductions in the mid to late 19th century, biologists used the replicate populations of known history to conduct natural experiments aimed at understanding how and why species evolve in novel environments. Most studies inferred microevolutionary processes by comparing founder and source populations for traits measured at a single point in time. Here I build upon this previous work by using temporally spaced genotypic, phenotypic, and geographic information to directly measure microevolutionary responses to changing environments. I first show using microsatellite DNA that founder effects were generally weak but geographically variable throughout the introduced range, and that genetic admixture potentially played a major role in the large-scale restructuring of genotypic and phenotypic variation in North America. I then show using spatial occurrence data that the house sparrow rapidly colonized areas of North America that were climatically similar to the native range, and that this swift colonization of potential niche space enabled empirical distribution models to accurately predict the subsequent continental expansion. Finally, I show using phenotypic measurements that wing length in introduced sparrow populations evolved by means of climatically induced bouts of directional selection, and that this adaptive response was quantitatively predicted by an optimized model of wing loading. Combined, the three retrospective studies contribute to a broader understanding of the proximate and ultimate factors that enable species like the house sparrow to evolve rapidly in new environments. The prospective benefit of these studies is to inform models aimed at predicting future change. |
