AboutZoos, Since 2008


A snap­shot of pup­fish evolution

pub­lished 11 Jan­u­ary 2013 | mod­i­fied 11 Jan­u­ary 2013

Pupfish-adaptive-landscapeChris Mar­tin has bred more than 3,000 hybrid fish in his time as a grad­u­ate stu­dent in evo­lu­tion and ecol­ogy at UC Davis, a pur­suit that has helped him cre­ate one of the most com­pre­hen­sive snap­shots of nat­ural selec­tion in the wild and demon­strated a key pre­dic­tion in evo­lu­tion­ary biol­ogy. A paper on the topic has been pub­lished in today’s issue of the jour­nal Sci­ence.

We can see a sur­pris­ingly com­plex snap­shot of nat­ural selec­tion dri­ving the evo­lu­tion of new spe­cialised species
Chris Mar­tin, Evo­lu­tion and Ecol­ogy depart­ment, UC Davis »

The “adap­tive land­scape” is very impor­tant for evo­lu­tion­ary biol­ogy, but rarely mea­sured, Mar­tin said. He’s been fas­ci­nated with the con­cept since high school. An adap­tive land­scape takes vari­able traits in an ani­mal or plant, such as jaw size and shape, spreads them over a sur­face, and reveals peaks of suc­cess (what evo­lu­tion­ary sci­en­tists call fit­ness) where those traits become most effec­tive, or adap­tive.

It is a com­mon and pow­er­ful idea that influ­ences think­ing about evo­lu­tion. But while the con­cept is straight­for­ward, it is much harder to map out such a land­scape in the wild. For exam­ple, about 50 species of pup­fish are found across the Amer­i­cas. The tiny fish, about an inch or so long, mostly eat algae on rocks and other detri­tus. Mar­tin has been study­ing species found only in a few lakes on the island of San Sal­vador in the Bahamas, where some of the fish have evolved different-​shaped jaws that allow them to feed on hard-​shelled prey like snails or, in one case, to snatch scales off other fish.

In a paper pub­lished in 2011, Mar­tin showed that these San Sal­vado­ran fish are evolv­ing at an explo­sively faster rate than other pup­fish. Mar­tin brought some of the fish back to the lab at UC Davis and bred hybrids with fish with dif­fer­ent types of jaws. He cre­ated about 3,000 hybrids in all, which were mea­sured, pho­tographed and tagged. Mar­tin then took about 2,000 of the fish back to San Sal­vador.

“It was the cra­zi­est thing I’ve done,” Mar­tin said. “I was lean­ing on the stack of them in the mid­dle of Miami air­port.”

Mar­tin released the young fish into enclo­sures in the lakes of their grand­par­ents. Three months later, he returned to check on the sur­vivors and plot­ted them out on the adap­tive land­scape. Most of the sur­viv­ing fish were on an iso­lated peak adapted to a gen­eral style of feed­ing, with another peak rep­re­sent­ing fish adapted for eat­ing hard-​shelled prey. Com­pe­ti­tion between the fish had elim­i­nated the fish whose jaws put them in the val­leys between those peaks. The scale-​eating fish did not sur­vive.

The results explain why most pup­fish species in Amer­ica have pretty much the same diets, Mar­tin said. The gen­er­al­ists are essen­tially stranded on their peak — vari­ants that get too far out fall into the val­ley and die out before they can make it to another peak. “It’s sta­bil­is­ing selec­tion,” he said. An early burst of vari­a­tion when fish entered a new envi­ron­ment with lit­tle com­pe­ti­tion could have allowed the shell-​eaters and scale-​eaters to evolve on San Salvador.

The above news item is reprinted from mate­ri­als avail­able at UC Davis. Orig­i­nal text may be edited for con­tent and length.
(Source: UC DAVIS News, 10.01.2013)

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