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Abstract. We are concerned with the climatic factors that affect the adaptive radiation of species into and within the temperate zone. As one progresses northward, there is a decrease in summer temperatures and a concomitant increase in the duration and intensity of winter cold. For the pitcher-plant mosquito, Wyeomyia smithii, it has been shown previously that range expansion and evolution has proceeded from south to north, and (2) either realistically hot summer temperatures or a simulated 7-mo winter reduce fitness by 60% or more in two New Jersey (40[degrees] N) populations. Herein, we compare the relative fitness of nine populations of W. smithii from 30[degrees] to 50[degrees] N when exposed to these same stressful summer temperatures and simulated winter.

We determined the survivorship, fecundity, and fertility of five cohorts from each population at each temperature in both environments, and we calculated the cohort replacement rate ([R.sub.0]) as the product of these three component traits. Surv ivorship declined with increasing latitude in the summer, but not the winter, environment. Fecundity was not correlated with latitude in either environment. Fertility declined with increasing latitude in both environments. [R.sub.0] was not correlated with latitude in either environment.

Hence, the interpretation of the adaptive responses to climate can depend critically on the trait being examined. Because [R.sub.0] is a composite index of fitness that includes any trade-offs among its constituent traits, we place greater reliance on its noncorrelation with latitude. Therefore, we conclude that, despite their clear impact on fitness, summer heat and winter cold have had little impact on the adaptive evolution of W. smithii to the climatic gradient of North America. The decline in summer heat and increase in the duration and intensity of winter cold as one moves north also impose a latitudinal gradient in the length of the favorable growing season. Many plants and animals use daylength to cue the seasonal eve nts in their life cycles. We then argue that the timing of seasonal development mediated by photoperiodic response constitutes the most immediate adaptation of populations to novel temperate climates, and that adaptation of thermal responsiveness to summer heat or winter cold takes place over a longer time scale or taxonomic distance.