Abstract
The Threespine Stickleback is ancestrally a marine fish, but many marine populations breed in fresh water (i.e., are anadromous), facilitating their colonization of isolated freshwater habitats a few years after they form. Repeated adaptation to fresh water during at least 10 My and continuing today has led to Threespine Stickleback becoming a premier system to study rapid adaptation. Anadromous and freshwater stickleback breed in sympatry and may hybridize, resulting in introgression of freshwater-adaptive alleles into anadromous populations, where they are maintained at low frequencies as ancient standing genetic variation. Anadromous stickleback have accumulated hundreds of freshwater-adaptive alleles that are disbursed as few loci per marine individual and provide the basis for adaptation when they colonize fresh water. Recent whole-lake experiments in lakes around Cook Inlet, Alaska have revealed how astonishingly rapid and repeatable this process is, with the frequency of 40% of the identified freshwater-adaptive alleles increasing from negligible (∼1%) in the marine founder to ≥50% within ten generations in fresh water, and freshwater phenotypes evolving accordingly. These high rates of genomic and phenotypic evolution imply very intense directional selection on phenotypes of heterozygotes. Sexual recombination rapidly assembles freshwater-adaptive alleles that originated in different founders into multilocus freshwater haplotypes, and regions important for adaptation to freshwater have suppressed recombination that keeps advantageous alleles linked within large haploblocks. These large haploblocks are also older and appear to have accumulated linked advantageous mutations. The contemporary evolution of Threespine Stickleback has provided broadly applicable insights into the mechanisms that facilitate rapid adaptation.