NATURAL SELECTION FOR ENVIRONMENTALLY INDUCED PHENOTYPES IN TADPOLES

Models suggest that phenotypic plasticity is maintained in situations where the optimal phenotype differs through time or space, so that selection acts in different directions in different environments. Some empirical work supports the general premise of this prediction because phenotypes induced by a particular environment sometimes perform better than other phenotypes when tested in that environment. We have extended these results by estimating the targets of selection in Pseudacris triseriata tadpoles in environments without predators and with larval Anax dragonflies. Tadpoles displayed significant behavioral and morphological plasticity when reared in the presence and absence of nonlethal dragonflies for 32 days in cattle tanks. We measured selection in the absence of free predators by regressing growth and survival in the tanks against activity and several measures of tail and body shape. We measured selection in the presence of predators by exposing groups of 10 tadpoles to Anax in overnight predation trials and regressing the average phenotype of survivors against the number of tadpoles killed. Selection in the two environments acted in opposite directions on both tail and body shape, although the affected fitness components were different. In the presence of Anax, tadpoles with shallow and narrow body, deep tail fin, and wide tail muscle survived best. In the absence of free predators, tadpoles with narrow tail muscle grew significantly faster, and those with shallow tail fin and deep body grew somewhat faster. Activity was unrelated to survival or growth in either environment. Developmental plasticity in tail shape closely paralleled selection, because tail fin depth increased after long-term exposure to Anax and tail muscle width tended to increase. In contrast, there was no plasticity in body shape in spite of strong selection for decreasing body depth. Thus, when confronted with a dragonfly predator, P. triseriata tadpoles adjusted their tail shape (but not body shape) almost exactly in the direction of selection imposed by Anax. These results suggest that phenotypic plasticity in some morphological traits, such as tail depth and tail muscle width, has evolved under intermittent selection by dragonflies. Other traits that undergo selection by dragonflies, such as body morphology, appear developmentally rigid, perhaps because of historically strong opposing selection in nature or other constraints.