Contemporary evolution in threespine stickleback from uplifted islands in Alaska

Abstract

How rapidly can evolution occur in the wild? Threespine stickleback (Gasterosteus aculeatus) fish are a prime model organism to address this question because we can study post-glacial populations that are only about 13,000 years old. During this relatively short period of time, oceanic stickleback have repeatedly and independently colonized newly available freshwater habitat, giving rise to resident freshwater populations that exhibit repeated patterns of phenotypic and genetic divergence. However, it is currently unknown whether it actually takes thousands of years for resident freshwater populations to diverge from the ancestral form. To address this question, we have identified phenotypically variable stickleback populations in freshwater sites formed on islands in Prince William Sound and the Gulf of Alaska as a result of uplift from the 1964 Alaska Earthquake. Population genomics analyses support the hypothesis of ongoing independent colonization of freshwater habitats by oceanic ancestors. Despite recurrent gene flow between oceanic and freshwater stickleback, we find that the magnitude of phenotypic and genetic divergence between the ancestral and derived populations is comparable to what has been observed in populations that were founded thousands of years ago. Our data implicate natural selection as the major driver of phenotypic diversification and support the hypothesis that the metapopulation organization of this species helps maintain a large pool of standing genetic variation available for selection when oceanic stickleback colonize fresh water. We propose that the greatest amount of phenotypic evolution occurs within the first few decades after stickleback colonize novel freshwater environments.