Despite Hawai‘i being praised as an unparalleled natural laboratory for the study of evolution, our understanding of evolutionary processes working below the water line remains in its infancy. Although there is no shortage of famous examples of endemic terrestrial Hawaiian taxa that show rampant speciation within lineages after colonizing the islands, there is only one known case of a marine organism diversifying in Hawai‘i.
Trends in the insular evolution of marine animals reflect fundamental differences in life history, particularly their dispersal. On land, low gene flow between islands and among disjunct habitats within islands has set the stage for many of these terrestrial taxa to undergo explosive speciation. In contrast, it is generally believed that marine colonizers produce only a single endemic species or population due to the lack of isolating barriers within the archipelago. So this leads us to ask the surprising question: do marine species really not diversify within Hawai‘i?
Addressing this question first requires that we test alternative modes of speciation where reproductive isolation can evolve without physical barriers to gene flow. A growing body of evidence supports the ecological speciation hypothesis, which predicts that natural selection can override gene flow and drive even adjacent populations along different evolutionary pathways. Ecological divergence has been largely ignored as an important mechanism in marine speciation, due primarily to the lack of convincing examples. In the 80’s, UH Zoology Professors E. Alison Kay & Steve Palumbi singled out Hawaiian limpets (opihi) of the genus Cellana as a potential exception to the rule, and just recently, Chris Bird (a UH alumnus) along with other UH professors have provided compelling evidence that this lineage of limpets has, in fact, diverged within the archipelago.
Discovering additional diverging taxa and identifying the mechanisms by which they evolve reproductive isolation in insular systems will be crucial to reinforcing this potential paradigm shift and advancing our understanding of the evolution of marine organisms in Hawaii and elsewhere. My PhD research aims to make a significant contribution to this critical goal by outlining a novel case of ongoing adaptive speciation of a marine fish in Hawaiian waters.
The Arceye hawkfish (Paracirrhites arcatus) is a small, coral-dwelling reef fish with two distinct color morphotypes (See Figure) that co-occur throughout the Hawaiian Islands. Although, co-occurring on the same reefs, morphotypes have distinctly different habitat preferences. There is, however, overlap of habitat ranges, indicating that color morphs are not physically isolated and gene flow is certainly possible. As such P. arcatus provides an ideal model to examine the mechanisms that could drive the evolution of reproductive isolation even in the face of gene flow.
We anticipate that the resultant data from this research will provide the first empirical evidence of a marine fish diversifying in the Hawaiian Islands and the first case study of ongoing adaptive speciation in any insular marine organism. We expect that data generated from this research will improve our understanding of marine evolution, by highlighting the role natural selection plays in driving high biodiversity in the oceans.
Bird C, Holland B, Bowen BW, Toonen RJ (2011) Diversification of sympatric broadcast-spawning limpets (Cellana spp.) within the Hawaiian archipelago. Molecular Ecology 20:2128-2141.
Cowie R, Holland B (2008) Molecular biogeography and diversification of the endemic terrestrial fauna of the Hawaiian Islands. Philosophical Transactions of the Royal Society of London B Biological Sciences 363:3363-3376.
DeMartini E, Donaldson T (1996) Color Morph-Habitat Relations in the Arc-Eye Hawkfish Paracirrhites arcatus (Pisces: Cirrhitidae). Copeia 1996 (2):362-371.
Hourigan T, Reese E (1987) Mid-ocean isolation and the evolution of Hawaiian reef fishes. Trends in Ecology and Evolution 2:187-191.
Kay E, Palumbi S (1987) Endemism and evolution in Hawaiian marine invertebrates. Trends in Ecology and Evolution 2:183-186.
Schluter D (2009) Evidence for Ecological Speciation and Its Alternative. Science 323:737-741.
And here some pics of your TA Jon doing his research on the Arceye Hawkfish.