Environmental temperature can modify not only rates of ectotherm growth, development, and reproduction, but also, at the extremes, temperature can limit survivorship and ultimately fitness. We studied ectotherm populations from a latitudinal gradient to understand how alterations in the thermal environment (e.g., rapid climate change) may affect the persistence of populations experiencing stress-induced mortality. Populations of the mosquito, Wyeomyia smithii, from 30-50Њ N in North America were subjected to heat and cold stress based on observed field temperatures during the simulated passage of a typical 4-5 d warm-or cold-weather front. Cold but not heat tolerance corresponded to latitude of origin, reflecting previously observed patterns in year-long fitness. Both heat and cold stress resulted in an average of 44% mortality relative to unstressed controls but did not result in a significant loss of fitness (R 0 ) at the population level. We conclude that individuals most likely to survive during periods of stress are also the individuals most likely to make the greatest contribution to the next generation in the absence of stress. Since individual survivorship must, at some level, become limiting to population viability, these results imply that apparently viable populations may be pushed rapidly to extinction by a small increment in environmental stress.

Because mortality accumulates with age, Fisher proposed that the strength of selection acting on survival should increase from birth up to the age of first reproduction. Hamilton later theorized that the strength of selection acting on survival should not change from birth to age at first reproduction. As organisms in nature do not live in uniform environments but, rather, experience periodic stress, we hypothesized that resistance to environmental stress should increase (Fisher) or remain constant (Hamilton) from birth to age at first reproduction. Using the pitcher‐plant mosquito, Wyeomyia smithii, we imposed heat stress by simulating the passage of a warm‐weather front at different preadult and adult stages. Contrary to either Fisher or Hamilton, stress tolerance declined from embryos to larvae to pupae to adults. Consequently, reproductive value appears to have been of little consequence in the evolution of stage‐specific tolerance of heat stress in W. smithii.