Abstracts (first author)
Natural selection drives incipient divergence despite continuous gene flow in sympatric color morphs of a coral reef fish
The role that natural selection plays in initiating divergence-with-gene-flow remains controversial. Color polymorphisms offer rich opportunities to explore how selection can drive the evolution of reproductive isolation even in the face of gene flow. We integrate genetics, ecology, and behavior to outline how disruptive selection on color pattern in combination with assortative mating is driving divergence in sympatric color morphs of the arceye hawkfish (Paracirrhites arcatus) in Hawaii. First, ecological surveys show a strong correlation between phenotype & environment. Fine-scale microhabitat variation in visual backgrounds creates distinct niches that appear to favor alternative color patterns. Experimental tests are ongoing, but preliminary evidence shows reduced fitness in morphs found in contrasting habitats. Second, field observations of mating pairs indicate that, even in zones of overlap, fish are 10x more likely to pair with like morphs. Lab-based experiments of captive fish are currently being used to further explore the strength of assortative mating and its impact on reproductive isolation. Third, genome scans using 30 microsatellite loci show divergent patterns of genetic variation across morphs indicative of divergence-with-gene-flow; and outlier detection methods confirm three loci as candidates for positive selection. The strong association between genetic variants, color pattern, and ecological gradients all suggest that P. arcatus has evolved genetically-based alternative phenotypes that are adapting to spatially heterogeneous habitats despite continuous gene flow. We are currently using comparative genomics to discover regions showing signatures of natural selection and identify the underlying genetic basis of adaptive color pattern and correlated traits. This research outlines one of the few case studies of ongoing sympatric divergence in any marine fish and will help elucidate the role color polymorphism plays in promoting speciation.