Evolutionary responses to environmental stress by the pitcher-plant mosquito, Wyeomyia smithii

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Abstract
We performed truncation selection for increased fitness (rc) under conditions of chronic stress from the combined effects of low nutrients and high temperature, representative of extremes likely to be encountered in nature by the pitcher-plant mosquito, Wyeomyia smithii. We performed selection on geographical parental populations and their hybrids to determine whether hybridization would facilitate or constrain adaptation under our selection protocol. The stressful environment decreased fitness (rc) by 54% averaged across all populations relative to near-optimal conditions. After approximately 10 generations of selection under chronically stressful conditions, exactly one-half of the parental and one-half of the hybrid populations had gone extinct. Thus hybridization had no effect on the likelihood of population persistence. Fitness (rc) of the surviving populations did not show any response to selection. Despite initial hybrid vigour under stressful conditions, the fitness (rc) of surviving hybrid populations was either equal to, or worse than, the fitness (rc) of surviving parental populations after approximately 10 generations of selection. These results suggest that outcrossing populations to augment genetic variation and facilitate adaptation to a rapidly changing environment may not be useful over longer time scales, even in cases where hybridization does initially increase fitness. Although we detected no direct response to selection for increased fitness (rc) under lifetime chronic stress, selected populations showed a strong correlated response for survivorship through transient, acute heat and desiccation shock. In evaluating how organisms might respond to future climate change, biologists must maintain a clear distinction between lifelong performance in chronically stressful environments and short-term survivorship through transient, acute stress.