COLLAR, D.C.*; University of California, Davis: Testing the bass fisherman�s hypothesis: Does reaching an adaptive peak limit diversification of the feeding apparatus in Micropterus (Teleostei: Centrarchidae)?

The potential for a lineage to diversify morphologically might be limited if it sits atop an adaptive peak. If the common ancestor of a lineage possesses structures and performance capacities that confer optimal fitness, then selection will act to limit the diversity of those structures in descendent lineages. Inspired by the observations of many bass fishermen that the black basses (Micropterus) are the ultimate fish predators, I tested the hypothesis that, with respect to the feeding mechanism, the position of Micropterus atop an adaptive peak has limited its diversity. Micropterus is a clade of eight freshwater fish species belonging to the North American radiation, Centrarachidae. The diets of all Micropterus species contain fish, and the diversity of diet items within this group is lower than that of its sister clade, Lepomis (sunfishes), and the sister clade to these two groups. Among prey items available to freshwater carnivores, fish contain the most protein and yield the highest growth rates. Thus, the Micropterus diet can plausibly be considered optimal among freshwater fishes. With respect to characters of the feeding apparatus, I found that Micropterus exhibits significantly lower within-clade variance on the first two principal components than the other two centrarchid clades. My hypothesis also predicts that attainment of an adaptive peak in a common ancestor of Micropterus is associated with a decrease in the rate of morphological evolution. I found that the first two principal components evolve more slowly in Micropterus than in the other two clades of the Centrarchidae. These results support the hypothesis that diversification of the Micropterus feeding mechanism has been restricted by its position at an adaptive peak.