RESUMEN
Species such as threespine stickleback (Gasterosteus aculeatus) that inhabit divergent selective environments and that have diversified on different time scales can be of value for understanding evolutionary processes. Here we synthesize high-resolution genotypic and phenotypic data to explore a largely unstudied distribution of threespine stickleback populations living in oceanic and freshwater habitats along coastal and inland regions of Oregon. Many inland aquatic habitats of Oregon remained unglaciated during the last ice age, meaning that some extant Oregon lake and river stickleback may have descended from freshwater populations established long before more well-studied, post-glacial freshwater populations. To address the degree of congruence between genetic and phenotypic divergence, we directly compared Oregon stickleback to much younger (post-glacial) Alaskan populations. We found phenotypic variation in Oregon stickleback to be primarily partitioned between oceanic and freshwater habitats, as has been documented in other stickleback systems. However, the main axis of genetic divergence was between coastal and inland regions regardless of habitat type. Furthermore, when comparing patterns between Oregon and Alaska we found similar levels of phenotypic divergence, but much greater genetic divergence among Oregon's populations. The Oregon stickleback system therefore appears well suited for future studies linking genotypic and phenotypic change, further extending the utility of this small fish to provide general insights into evolutionary processes.
RESUMEN
How rapidly can animal populations in the wild evolve when faced with sudden environmental shifts? Uplift during the 1964 Great Alaska Earthquake abruptly created freshwater ponds on multiple islands in Prince William Sound and the Gulf of Alaska. In the short time since the earthquake, the phenotypes of resident freshwater threespine stickleback fish on at least three of these islands have changed dramatically from their oceanic ancestors. To test the hypothesis that these freshwater populations were derived from oceanic ancestors only 50 y ago, we generated over 130,000 single-nucleotide polymorphism genotypes from more than 1,000 individuals using restriction site-associated DNA sequencing (RAD-seq). Population genomic analyses of these data support the hypothesis of recent and repeated, independent colonization of freshwater habitats by oceanic ancestors. We find evidence of recurrent gene flow between oceanic and freshwater ecotypes where they co-occur. Our data implicate natural selection in phenotypic diversification and support the hypothesis that the metapopulation organization of this species helps maintain a large pool of genetic variation that can be redeployed rapidly when oceanic stickleback colonize freshwater environments. We find that the freshwater populations, despite population genetic analyses clearly supporting their young age, have diverged phenotypically from oceanic ancestors to nearly the same extent as populations that were likely founded thousands of years ago. Our results support the intriguing hypothesis that most stickleback evolution in fresh water occurs within the first few decades after invasion of a novel environment.