The Effect of Rare and Novel Temperature Regimes on the Release of Cryptic Variation in the Fathead Minnow, Pimephales promelas
Schertzing, Claire L.
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Warmer temperatures brought about by climate change will have widespread effects on the physiology and life history of organisms. Importantly, populations will be increasingly exposed to novel environmental conditions. When a population is exposed to these extreme environments, theory suggests that hidden variation in phenotypes will be uncovered. Theoretically extreme environments should expose a wider range of phenotypes to selection than those seen under canalization. To determine the effect of normal, rare, and novel thermal conditions on the release of this hidden variation, we exposed fathead minnows, Pimephales promelas, to five fluctuating temperature regimes (22/25,22/28,22/31, 22/34, and22/37°C). Minnow growth was measured over the course of the experiment as well as CTmax, and food consumption. Mean growth decreased as temperatures increased, while variation in growth also decreased. CTmax, the highest temperature at which the fish can maintain equilibrium, increased as thermal regimes became warmer, but here again variation in the trait decreased. Food consumption was significantly different between all treatments with decreasing consumption as temperature increased. Decreased variation of all variables points to a narrowing rather than broadening of exposed phenotypes under extreme conditions. Loss of variation in populations exposed to rare and novel environments could be particularly disastrous for the future of species, as natural selection necessitates phenotypic variation to act on.