How repeatable is evolution at genomic and phenotypic scales? We studied the repeatability of evolution during 8 generations of colonization using replicated microcosm experiments with the red flour beetle, Tribolium castaneum. Based on the patterns of shared allele frequency changes that occurred in populations from the same generation or experimental location, we found adaptive evolution to be more repeatable in the introduction and establishment phases of colonization than in the spread phase, when populations expand their range. Lastly, by studying changes in allele frequencies at conserved loci, we found evidence for the theoretical prediction that range expansion reduces the efficiency of selection to purge deleterious alleles. Overall, our results increase our understanding of adaptive evolution during colonization, demonstrating that evolution can be highly repeatable, while also showing that stochasticity still plays an important role.

A number of studies have suggested a link between hybridization and invasion. In this study, we experimentally test the potential for hybridization to influence invasion through a greenhouse common garden study. Diffuse knapweed (Centaurea diffusa Lam.) was introduced to North America with admixture from spotted knapweed (Centaurea stoebe subsp. stoebe L.). Comparisons between North American diffuse knapweed (including hybrid phenotypes) and native (European) diffuse knapweed in a common garden did not reveal enhanced performance or increased phenotypic variance, suggesting that pre-introduction hybridization or, more generally, post-introduction evolutionary change, has not significantly contributed to the invasion of diffuse knapweed. In contrast, early generation hybrids [artificially created Back Cross 1 (BC1) plants] exhibited increased variance for eight of the examined traits, and greater leaf and reproductive shoot production when compared to North American diffuse knapweed. Individual BC1 lines differed for several traits, suggesting the importance of the cross for drawing conclusions from such comparisons. When compared to the parental species (diffuse and spotted knapweed), the BC1 plants were not transgressive for any of the measured traits. Overall, these findings suggest that if diploid spotted knapweed is introduced to North America, interspecific hybridization has the potential to result in even more aggressive invaders.