Adaptation and plasticity

For my Ph.D., I used brown trout (a sea-going juvenile shown above) introduced to the island of Newfoundland as a case study to examine the early stages of an adaptive radiation. I conducted a series of projects to examine population differentiation in the wild and assessed the genetic and environmental underpinnings, along with the adaptive significance, with common-rearing and  in situ reciprocal transplant experiments.  In short, results suggest that populations have divergence in a suite of phenotypic traits used to distinguished among populations in the native range and often assumed to be linked to fitness (Westley et al. in review).  In addition, reciprocal transplants suggest that local individuals out perform foreign invaders suggesting that adaptation has evolved in ca. 32 trout generations and at fine-spatial scales.

Phenotypic plasticity – the ability of an organism to respond to an environmental stimulus with a change in state, form, movement, or behaviour – is increasingly thought to represent a mechanism for populations and species to cope with abruptly changing environmental conditions. Over a hundred years ago, J. M. Baldwin proposed a ‘new factor’ in evolution whereby plasticity could facilitate survival in new environments and allow selection to act on the survivors in the direction of the plastic response. These predictions encapsulate the first vital stages of the ‘Baldwin effect’ whereby environmental induction of traits can shape the course of subsequent evolution; however, few studies have attempted to empirically test these predictions in nature. Although the ideas of Baldwin enjoy considerable theoretic support, few researchers have attempted to test predictions empirically.  To do so, I combined common-garden and reciprocal- transplant experiments along with formal quantifications of natural selection to test the hypotheses that phenotypic plasticity acting body size, growth rate, and functional morphology in juvenile brown trout (Salmo trutta) should allow individuals to persist when introduced to novel environments and that plasticity should be predictable based on patterns of natural selection. To do so, we raised individuals from three populations in common laboratory conditions until large enough to tag and track in the wild. We detected marked plasticity in swimming morphology, specifically the depth of the head and body, after approximately two months of rearing in three wild streams. Populations that survived introduction were generally consistent in their plastic responses, though we did detect evidence of population-specific plasticity suggesting underlying genetic variation. Counter to predictions, the plasticity we observed was frequently in the opposite direction from selection even though it moved in a direction generally assumed to be adaptive (i.e. small rivers seemed to plastically induce shallow-bodies and vice versa in large rivers). We did detect evidence of greater survival and growth of individuals reared in their local environments compared to when reared in foreign locations, suggesting local adaptation has evolved in these populations recently descended from common ancestors. Overall, our results suggest that plasticity may shape phenotypes in unpredictable ways and that attempts to forecast the response of populations to rapidly changing global environments may be prone to failure.

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