Pelvic Reduction in Threespine Stickleback from Cook Inlet Lakes: Geographical Distribution and Intrapopulation Variation

Abstract

Loss of one or more elements of the pelvic complex occurs at substantial frequencies (-5%) in 55 and at lower frequencies in 42 of 204 populations of Gasterosteus aculeatus sampled from freshwater sites around Cook Inlet, Alaska. Populations with substantial pelvic reduction are widely distributed and interspersed among those lacking it. Intrapopulation phenotype frequencies vary greatly within limited areas but may be similar in distant populations. Intrapopulation phenotype frequency distributions include bimodal, flat, normal, skewed, and truncated; and their form may vary among adjacent populations. High intrapopulation frequencies of pelvic reduction apparently have evolved repeatedly within Cook Inlet, but gene flow is probably important in spreading genetic variation for pelvic reduction among populations. Divergent populations of Cook Inlet threespine stickleback should be treated as parts of an endemic radiation, which warrants special consideration for conservation as a unit.