Adaptive plasticity generates microclines in threespine stickleback male nuptial color


Adaptive phenotypic divergence is typically studied across relatively broad spatial scales (continents, archipelagos, river basins) because at these scales we expect environmental differences to be strong, and the homogenizing effect of gene flow to be weak. However, phenotypic plasticity and phenotype-dependent habitat choice are additional mechanisms that could also drive adaptation across spatially variable environments. We present evidence for apparently adaptive phenotypic variation across surprisingly small spatial scales (<2 vertical meters) in the threespine stickleback. We find that male breeding coloration varies as a function of the lakes’ optical-depth gradient, and these small-scale clines (‘microclines’) appear to be an adaptive response to ambient light gradients, as male color changes predictably in the opposite direction (‘countergradient’) to ambient light spectral shifts. Using visual models and field enclosure experiments, we show that these microclines result from phenotypic plasticity that maintains male conspicuousness. Our results show that adaptive phenotypic clines can exist across small spatial scales, because phenotypic plasticity rapidly generates repeatable trait-environment correlations despite the overwhelming opportunity for gene flow. Furthermore, these results provide strong evidence that phenotypic plasticity in nuptial coloration is an important mechanism for adjusting the conspicuousness of a visual signal to conspecifics.

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