A Single Trait Drives Incipient Ecological Speciation in Sympatric Color Morphs of the Arceye Hawkfish

Abstract

Coral reef fishes represent the most diverse assemblage of vertebrates on the planet, yet our understanding of the mechanisms driving this diversity remains limited. There is growing recognition that ecological adaptation shaped by natural selection may be a major driver of diversification on coral reefs. However, few examples of ecological speciation in nature currently exist. I integrate research on ecology, behavior and genetics to outline a novel case of incipient ecological speciation in sympatric color morphs of the arceye Hawkfish (Paracirrhites arcatus). First, I demonstrate that color morphs are exploiting different niches along a steep ecological gradient, likely driven by disruptive selection favoring color patterns that are better camouflaged in contrasting microhabitats. Second, mate preference experiments show that females prefer individuals of their own morph, indicating color morphs are mating assortatively. Third, I provide genetic evidence that these premating barriers have resulted in at least partial reproductive isolation between ecologically differentiated sympatric color morphs. Taken together, these results suggest that reproductive isolation between morphs may be arising as a by-product of divergent selection on ecological differences and enhanced by the isolating effects of assortative mating. I conclude that color alone is driving incipient divergence in this species, despite high gene flow and no geographic isolation. I argue that the characteristics of this system could be quite common and thus widely applicable to thousands of reef organisms. This dissertation emphasizes the role natural selection plays in initiating speciation and should help bring us one step closer to understanding the processes driving high biodiversity in tropical seas.