Effects of colonization history and landscape structure on genetic variation within and among threespine stickleback (Gasterosteus aculeatus) populations in a single watershed

Question: How do post-glacial colonization history and current watershed geomorphology affect population genetic structure in lacustrine fish populations? Study system: Eleven populations of lacustrine threespine sticklebacks (Gasterosteus aculeatus) from a single watershed, and two adjacent marine populations of G. aculeatus as outgroups. Methods: Individuals were genotyped using six microsatellite loci and several population genetic parameters were calculated (including genetic differentiation, genetic diversity, migration, and divergence time). We regressed population genetic parameters against environmental variables such as stream gradient and length, distance from the mouth of the watershed, and lake area. Results: Genetic differentiation was highest between lakes separated by high-gradient streams, but was unaffected by stream length. Estimated divergence times between adjoining populations declined from >10,000 years ago for lake pairs close to the ocean to less than 5000 years for lake pairs near the top of the watershed. Genetic diversity declined as a function of geographic distance from the mouth of the watershed but was positively correlated with lake area. Conclusions: Our findings confirm expectations that landscape features such as stream gradient and stream branching structure strongly influence patterns of genetic divergence. However, in contrast to previous studies, we found no significant relationship between geographic distance and genetic divergence among lake pairs. We present novel evidence that post-glacial colonization occurred gradually over a significant period of time, rather than a single rapid invasion into all lakes.