Variation in feeding morphology between pumpkinseed populations: Phenotypic plasticity or evolution?

Pumpkinseed sunfish exhibit considerable intraspecific variation in jaw mor phology, with population-level differences in the size of key morphological structures often exceeding 200%. This inter-population variation is correl ated with differences in the availability of gastropods, the pumpkinseed's primary prey. Such resource polymorphisms may be an indication of local ada ptation to resource conditions. This explanation, however, assumes that the observed phenotypic variation has an underlying genetic basis. Here, we pr ovide evidence from two types of common-garden experiments that variation i n pumpkinseed pharyngeal jaw morphology is the result of phenotypic plastic ity We collected adult fish from two populations (with the greatest observed di vergence in morphology and resource use), bred them in similar environments , and then raised their young under two conditions: (1) the laboratory wher e we controlled diets and fed fish either soft-bodied prey only, or a combi nation of soft-bodied prey and gastropods; and (2) experimental ponds in wh ich the fish were unconstrained with respect to diet. After 1 year, we anal ysed the pharyngeal morphology of the fish, focusing on the size of the lev ator posterior, the primary muscle used to generate the force needed to cru sh snail shells. In the laboratory, there were no differences in morphology attributable to parental stock under either dietary treatment. In contrast , the addition of snails to the fishes' diets led to a substantial (similar to 230%) increase in levator posterior mass. In the ponds, we observed som e slight, and inconsistent, differences between populations with respect to muscle mass. In all cases, the population-level differences were very smal l compared to the effects of different ponds, or compared to the difference s in morphology observed in the laboratory. These results show that the nat ural variation in pharyngeal morphology between these populations of pumpki nseeds is primarily the result of a plastic response to the environment, ra ther than a response to selection driven by the environmental differences.