| Interspecific resource competition is thought to cause frequency-dependent selection to favor phenotypes least like the competitor, resulting in ecological character displacement (ECD). ECD causes the divergence of ecologically similar, coexisting species. I use comparative analyses and manipulative experiments to test for ECD in brook sticklebacks that coexist with ninespine sticklebacks in a series of independent post-glacial lakes in northern Ontario. I found brook sticklebacks to be divergent in body form where they coexisted with ninespines compared to where they lived without ninespines. Frequency-dependent selection favored the divergent brook stickleback body form in the presence of ninespines. Shifts in resource use were related to shifts in body form; this specialization conferred a fitness advantage for brook sticklebacks foraging on benthic prey, as opposed to pelagic prey, as expected. I conclude that there is strong evidence consistent with the hypothesis that ECD has played a role in the divergence of brook sticklebacks in this system. |
