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

Pumpkinseed sunfish exhibit considerable intraspecific variation in jaw morphology, with population-level differences in the size of key morphological structures often exceeding 200%. This inter-population variation is correlated with differences in the availability of gastropods, the pumpkinseed's primary prey. Such resource polymorphisms may be an indication of local adaptation to resource conditions. This explanation, however, assumes that the observed phenotypic variation has an underlying genetic basis. Here, we provide evidence from two types of common-garden experiments that variation in pumpkinseed pharyngeal jaw morphology is the result of phenotypic plasticity We collected adult fish from two populations (with the greatest observed divergence in morphology and resource use), bred them in similar environments, and then raised their young under two conditions: (1) the laboratory where we controlled diets and fed fish either soft-bodied prey only, or a combination of soft-bodied prey and gastropods; and (2) experimental ponds in which the fish were unconstrained with respect to diet. After 1 year, we analysed the pharyngeal morphology of the fish, focusing on the size of the levator posterior, the primary muscle used to generate the force needed to crush 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 some slight, and inconsistent, differences between populations with respect to muscle mass. In all cases, the population-level differences were very small compared to the effects of different ponds, or compared to the differences in morphology observed in the laboratory. These results show that the natural variation in pharyngeal morphology between these populations of pumpkinseeds is primarily the result of a plastic response to the environment, rather than a response to selection driven by the environmental differences.