Geographic variation in male nuptial color across multiple scales in threespine stickleback, Gasterosteus aculeatus



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Most species exhibit geographic variation in phenotypic traits, and this variation can occur across a broad range of spatial scales. Consistent phenotype-environment correlations are often invoked as evidence for adaptation in response to spatially varying natural selection. Both theoretical and empirical work suggests that local adaptation can occur most readily when the strength of selection exceeds the homogenizing effect of gene flow. Consequently, adaptive phenotype-environment correlations are typically studied at broad spatial scales at which gene flow is weak and environmental variation is pronounced. Despite this, a number of species show adaptive phenotypic divergence across microspatial scales. Signal detectability in animal communication is frequently contingent on the local environment. Consequently, to maintain optimal detectability, signal design should covary with the signaling environment, resulting in predictable phenotype-environment correlations. We investigated whether spatial variation in nuptial color of male threespine stickleback, Gasterosteus aculeatus, varies predictably with optical environment at two spatial scales: 1) between lakes 2) within lakes. We demonstrate that male nuptial color varies significantly between 15 lakes on Vancouver Island, BC. Male orange/red reflectance negatively covaries with the amount of orange/red in the optical background, suggesting that nuptial color tracks the local optical environment to maintain signal contrast. However, visual model results indicate that male signal contrast varies between lakes. Some lakes showed evidence of local adaptation, while others did not. A number of ecological characteristics also covary with male color and contrast, and we suggest these factors may constrain males’ ability to track local signaling optima. Within lakes, we found ’microclines’ across a small spatial gradient: male nuptial color changes across a 2-meter vertical range of nest depths. The color microcline is repeatably associated with depth gradients in ambient light, suggesting that these microclines reflect adaptation to local optical environments. Visual modeling demonstrates that these signals vary in contrast with background, depending on a male’s nest depth. Deeper-nesting males reflected more UV/blue and were more conspicuous than shallow-nesting males. Experimental manipulation of male nesting depth induced plastic changes in nuptial color that replicated the natural gradients in color and conspicuousness, implicating plasticity as the primary mechanism driving microcline formation.