VIABILITY SELECTION ON SEASONALLY POLYPHENIC TRAITS: WING MELANIN PATTERN IN WESTERN WHITE BUTTERFLIES.

Abstract

Wing melanin pattern varies seasonally among generations in many populations of the butterfly Pontia occidentalis, leading to distinctly different wing phenotypes during spring and summer generations. Estimates of directional selection on wing pattern can therefore quantify the imperfection of this phenotypically plastic (polyphenic) response in generating "optimal" phenotypes for each seasonal generation. Mark-release-recapture (MRR) studies were used to estimate directional selection on six wing traits in a natural population of P. occidentalis during both spring and summer weather conditions. Estimated survival and recapture probabilities varied substantially among the four MRR studies. When differences between males and females were detected, the survival and recapture probabilities were higher for males than for females. Estimated selection coefficients suggested that the direction of selection on one wing trait important for thermoregulation, melanin on the base of the dorsal hindwings (trait hb), fluctuated seasonally; there was evidence of directional selection for increased hb in the spring studies and for decreased hb in the summer studies. Such fluctuating seasonal selection on hb implies that the seasonal polyphenic response may not be sufficient to eliminate selection on this trait; the slope of the reaction-norm mapping hb onto seasonal environmental cues is too shallow, resulting in further selection on the reaction norm. Adaptive evolution of the reaction norm may be constrained by phenotypic and genetic correlations with other wing traits that experience different patterns of selection and by variable weather conditions within seasons and among years.