Meeting Abstract

32.7  Tuesday, Jan. 5  Contingency and determinism in the trophic apparatus of threespine stickleback: implications for adaptive evolution MCGEE, MD*; WAINWRIGHT, PC; Univ. of California, Davis; Univ. of California, Davis mcgee@ucdavis.edu

Adaptive evolution is often thought to proceed in a deterministic fashion, with chance and contingency playing only a minor role as the population ascends to a new phenotypic optimum. However, if selection acts on an index trait that combines several phenotypic traits into one functional trait, then ascension to a new adaptive peak may produce similar index trait values even if they are derived from different phenotypic combinations. To test this, I cleared and stained specimens of threespine stickleback from British Columbia and Alaska belonging to both the “limnetic” and “benthic” ecotypes. For each fish, I measured the components of the suction index model, which predicts the maximum suction pressure generated during a feeding event. Limnetics from Alaska and British Columbia had a low suction index consistent with their preferred diet of evasive zooplankton, and benthics from the two locales had a high suction index consistent with feeding on epi- and infaunal invertebrates. Limnetic-benthic differences within British Columbia populations related mostly to epaxial muscle cross-sectional area, with benthics having significantly higher CSA than limnetics. Suction index in Alaskan fish differed primarily due to size of the buccal cavity, with benthics having a significantly shorter buccal length and a smaller gape than limnetics. Despite the fact that each ecotype had similar values of suction index when compared to other populations of that same ecotype, the underlying phenotypes of each population were highly divergent. This result suggests that for complex traits, evolution to a new adaptive optimum can proceed in a deterministic fashion with an underlying diversity of outcomes due to chance and contingency.