Contemporary evolution, allelic recycling, and adaptive radiation of the threespine stickleback

Abstract

Background: Adaptive radiation in the threespine stickleback (Gasterosteus aculeatus) is dramatic, highly replicated, and predictable. Goals: Test hypotheses that divergent phenotypes of freshwater threespine stickleback evolve at the same rate as in other species and that derived freshwater and ancestral oceanic (i.e. marine or anadromous) phenotypes are genetically additive. Update a model for contemporary evolution and adaptive radiation based on new genetic and genomic findings. Methods: Summarize published information on contemporary evolution of threespine stickleback and compare published stickleback rates of evolution to those of other species. Analyse F1 hybrid and pure crosses to estimate the coefficient of genetic dominance of pheno-types that differ between oceanic and freshwater populations. Review published information on the genetics and genomics of stickleback phenotypes. Results: Threespine stickleback populations that experience large environmental changes may evolve measurably for multiple traits within ten generations. Rare freshwater-adapted alleles have been recycled from freshwater to oceanic populations by introgression and increase to high frequencies when oceanic stickleback colonize fresh water. These freshwater-adapted alleles tend to be partially recessive, to produce adaptive phenotypic plasticity, and to be linked within several genomic regions, which facilitates their retention in oceanic populations and the evolutionary response to directional selection after invasion of fresh water. Conclusion: The metapopulation structure and great age of the threespine stickleback have produced a genomic architecture and abundant allelic variation that are conducive to pre-dictable contemporary adaptive radiation involving numerous genes and phenotypic traits.