Big Island fish evolving without geographic barriers: this is strange stuff

Arc-eyed Hawkfish, this one from Fiji in 2008. 

Credit: NOAA photo by Julie Bedford

If you isolate populations of animals and plants long enough, they can evolve into different forms, even different species.

That's been known for a long time.

In the Hawaiian Islands, we also have lots of evidence that the isolation doesn’t require long distances. A plant or insect in one steep-sided valley can have evolved into a unique species from its relatives in the next valley.

The valley itself may be sufficient to isolate the genetic flow, and allow each group to evolve independently.

But can species isolate themselves without geographic barriers? Apparently so, and you can find examples on Hawaiian reefs.

Researchers Jonathan Whitney, Brian Bowen and Stephen Karl, all of the Hawai`i Institute of Marine Biology studied arc-eye hawkfish (Paracirrhites arcatus) off the Big Island, where they found dark-colored fish on basalt bottoms and light-colored fish in coral habitats—all within a few feet of each other.

And it turned out that the dark colored hawkfish were more closely genetically related to dark hawkfish far away than they were to their light-colored cousins nearby. The fish apparently were isolating themselves voluntarily by their preferred habitat.

Whitney, Bowen and Karl published their research in the journal Molecular Ecology, under the title, "Flickers of speciation: Sympatric colour morphs of the arc-eye hawkfish, Paracirrhites arcatus, reveal key elements of divergence with gene flow." 

They wrote: "We observed greater genetic divergence between colour morphs on the same reefs than that between the same morphs in different geographic locations. We hypothesize that adaptation to the contrasting microhabitats is overriding gene flow and is responsible for the partial reproductive isolation observed between sympatric colour morphs."

Apparently, the light-colored fish on coral select their mates from among the other light-colored fish on coral, rather than from among the dark-colored fish on the basalt a short distance away. And vice versa.

"The combination of ecological, behavioural and now genetic  studies of the arc-eye hawkfish provides compelling evidence for partial reproductive isolation resulting from ecological barriers in the absence of geographic isolation."

The hawkfish have not been sufficiently isolated to have developed into separate species, but they seem to be on their way in that direction. And that's both interesting and strange, but may be a piece to a puzzle, the authors write:

"Whether complete reproductive isolation will develop between arc-eye colour morphs remains speculation. Regardless of the outcome, P. arcatus provides a rare case confirming that partial reproductive isolation can evolve in the face of continuous gene flow, bringing us one step closer to understanding the role ecological barriers play in initiating the early stages of speciation."

©Jan TenBruggencate 2018