Evolutionary origins of genomic adaptations in an invasive copepod.

The ability of populations to expand their geographical ranges, whether as invaders, agricultural strains or climate migrants, is currently one of the most serious global problems. However, fundamental mechanisms remain poorly understood regarding factors that enable certain populations, such as biological invaders, to rapidly transition to novel habitats. According to one hypothesis, environmental fluctuations in the native range could promote successful invasions by imposing balancing selection on key traits and maintaining the genetic variation that enables rapid adaptation in novel habitats. Here we test the genomic predictions of this hypothesis by performing whole-genome sequencing of multiple independent invasive freshwater and native saline populations of the copepod Eurytemora affinis complex. We found that invasive populations have repeatedly responded to selection through the parallel use of the same single-nucleotide polymorphisms and genomic loci, to a much greater degree than expected. These same loci were enriched for signatures of long-term balancing selection in the native ranges, with 15-47% of loci exhibiting significant signatures of balancing selection. The strong association between parallel evolution in the invaded range and balancing selection in the native range supports the hypothesis that fluctuating habitats can promote invasive success and that balancing selection might serve as a widespread and important mechanism that enables rapid adaptation in nature.