Fishy research exposes rapid evolution

10/1/2012

Evolution and Ecology graduate student Chris Martin’s primary field lab is on the island of San Salvador in the Bahamas. There, in an extremely rare twist of evolution, he has discovered that a tiny saline lake is home to three unique species of pupfish—an algae eater, a snail and shrimp eater, and one that survives solely by eating the scales of the other two.

About 50 species of pupfish (genus Cyprinodon) are found from Massachusetts to Venezuela—and Martin said they all pretty much “look the same and act the same,” eating detritus and algae off rocks.

So Martin, who works with Evolution and Ecology Professor Peter Wainwright, was astounded to identify these three unique varieties swimming in such close quarters. To study how and why the fish evolved so distinctly, he devised a novel experiment: He bred a “common ancestor” fish by crossing the three species at his campus lab.

Then he flew to the Bahamas with 3,000 live pupfish. “It wasn’t your typical carry-on,” he said. “I was so relieved when they landed alive and well.”

Back at the lake, Martin built an enclosure within the water and seeded it with his hybrids. He left them there to live for a field season.

Upon collection, Martin analyzed the hybrids’ offspring for phenotypical changes indicating speciation. He created “fitness landscapes” for these mutations, graphs that chart the relationship between phenotypes and survival.

The landscapes showed two distinct peaks in fitness corresponding to multiple traits. Hybrids on one peak resembled the phenotype of the algae-eating species observed in the wild. Hybrids on the second peak resembled the phenotype of the snail and shrimp eater.

One of the major traits defining these two peaks was 'nose' length: The snail-and-shrimp eater has a novel nasal appendage which may help it to crush its hard-shelled prey. Hybrids with a large 'nose' survived best, intermediate 'nose' survived poorly, and lack of a 'nose' survived well.

His dynamic presentation of the results at the 2012 SSE meeting earned him the W. D. Hamilton Award, given to “a student who has presented an outstanding talk at the annual meeting.” He will share the $1,000 prize with Aleeza Gerstein of the University of British Columbia.

Martin’s initial research on the speciation in this lake—specifically his discovery of the scale-eating pupfish—was published online in the journal Evolution in 2011. No other pupfish is known to eat scales, Martin said.

“I was stunned to find only scales and no whole fish when I began examining the guts of this species,” Martin said. “This behavior is easy to watch in the field—the scale-eater stalks any nearby pupfish, quickly orienting perpendicular to its prey, striking and biting off scales, then stealthily moving on to the next target, just like a pup-tiger.”

In the 2011 paper, Martin described how pupfish in San Salvador and at another saline lake in the Yucatan evolved changes to their jaws to match their diets, allowing him to construct an evolutionary map for the species.

If the evolution of all pupfish is like a steadily expanding cloud, Martin found that the San Salvador Island and Yucatan pupfish are like bursts of fireworks within it. They show explosive rates of evolution—changing up to 130 times faster than other pupfish, he said.

It’s not clear why the pupfish in the two locations are evolving so fast. In both places, the lake water is hot and salty — but that’s true in other places where pupfish live. And mosquito fish, found in the same two lakes, show no signs of rapid change. Martin plans to continue his research to answer these evolutionary mysteries.