Interaction between predation environment and diet constrains body shape in Utah chub, Gila atraria (Cypriniformes: Cyprinidae)

Abstract

Evolution typically occurs in response to a suite of selective pressures. Yet, many studies of natural selection in the wild only investigate a single selective agent at a time. This can be problematic when selective agents act in non-additive ways. Here we evaluate the interactive effects of diet and predation on the evolution of body shape in the cyprinid fish Utah chub (Gila atraria). We found that both factors and the interaction between them are significant predictors of body shape. This interaction is likely a result of different forms of selective pressures, where predation is a stabilizing selective force and diet is a disruptive selective force. Utah chub with more herbivorous diets exhibited a distended abdomen and shorter, shallower caudal peduncle relative to Utah chub with more carnivorous diets. These shapes correspond with common patterns of ecological divergence between limnetic and benthic morphotypes, and likely evolved due to diet specialization. Utah chub from predation environments are generally more streamlined and have larger caudal peduncles than Utah chub in non-predation environments, which may be an adaptive response to allow greater burst-swimming capability. However, Utah chub in predation environments also have deeper bodies than more-carnivorous Utah chub in non-predation environments. Therefore, Utah chub in predation environments exhibit an intermediate phenotype with an intermediate depth but larger caudal peduncle. Our results suggest that predation constrains the range of body shape variation that is expressed in response to diet. Hence, the interactive effects of multiple selective pressures appear to be important in determining overall phenotype.