Life-history evolution in the pitcher-plant mosquito, Wyeomyia smithii

Long-term fitness in a seasonal environment depends on the ability to exploit favorable conditions with demographic life-history tactics, to avoid unfavorable conditions with phenological life-history tactics, and to convert between these two tactics. The pitcher-plant mosquito, Wyeomyia smithii, cues its seasonal conversion between development and dormancy with photoperiod.; To determine how genetic variation affects divergence in the switchpoint or critical photoperiod along a seasonal, latitudinal gradient, I estimated the components of genetic variation in this trait within and between two ancestral and four progressively more derived populations. Parent-offspring regression showed that the additive variance in critical photoperiod increased exponentially from ancestral to derived populations. Joint scaling tests of reciprocal first- and second-generation hybrids between these populations revealed significant and substantial epistatic effects on population divergence in critical photoperiod. The increase in additive variance in derived populations has occurred despite consistent directional selection for longer critical photoperiod in these populations. I argue that this increase in additive variance has not resulted from fluctuating selection or mutation-selection balance. Rather, I propose that this increase is the consequence of multiple founder events that have released additive effects from epistatic interactions during the repeated, northward divergence of W. smithii.; Critical photoperiod tracks latitude tightly but development time, a demographic trait, does not. During the range expansion of W. smithii, these traits have therefore evolved independently. The correlated response of critical photoperiod to direct selection on development time in the six populations revealed significant genetic correlations between these two traits. However, the genetic correlation varied in both sign and magnitude among populations and was not correlated with latitude. Genetic correlations may therefore affect the immediate response to selection within a population but genetic correlations do not necessarily impede the longer-term genetic differentiation of conspecific populations. Despite W. smithii's tight host specificity and remarkably consistent microhabitat throughout its geographic range, local adaptation has been more important than adaptation to geographical gradients of seasonality or density-dependent development in determining the genetic architecture of demographic and phenological life-history tactics.