Contrasting Size Distributions among the Wing Spots of a Pierid Butterfly Suggest Different Selective Histories.

Males of the small cabbage butterfly Pieris rapae crucivora have two dark or melanic spots in the central white area of each dorsal forewing, an anterior spot (aS) and a posterior spot (pS). We used characteristics of the size distributions of these spots to infer how selection has acted on them during their evolution. Our study reveals that the aS size distribution is normal while that of pS is very right-skewed. Moreover, aS size is larger and less variable than pS size. These results suggest that the aS has been under stabilizing selection while the pS has not. The context in which this selection is acting is not yet clear, nor is it clear why pS persists as a wing marking.