Genetic coordination of demography and phenology in the pitcher-plant mosquito,Wyeomyia smithii

Demographic and phenological traits compose the basic elements of an insect's life-history strategy in a seasonal environment. An insect's long-term fitness depends on its ability to exploit favorable conditions, to avoid unfavorable conditions, and to convert from one life style to the other. For the pitcher-plant mosquito, Wyeomyia smithii, we show that genetic variation exists in both development time (a demographic trait) and critical photoperiod (a phenological trait) in six populations spanning much of the species' geographical range. During the northward range expansion of W. smithii in North America, these traits have evolved independently under strong directional and stabilizing selection. The correlated response in critical photoperiod to divergent selection on development time reveals significantly positive genetic correlations in five populations and a negative correlation in one population. The positive correlations form a genetically coordinated phenotype: faster developing individuals use a shorter photoperiodic switch point and are able to exploit the late favorable season; slower developing individuals use a longer photoperiodic switch point and are able to avoid extending development into the unfavorable season. This genetic coordination of demography and phenology has not, however, prevented their independent evolution. We propose that evolutionary flexibility in W. smithii may arise in part from the reorganization of their genetic architecture following repeated founder events during their northward invasion of North America.