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Rock-Paper-Lizard

blog_slug_LizardUCSC researchers use game to explain findings on speciation

What does the game of rock-paper-scissors have in common with speciation? More than you may think, according to a paper by researchers at the University of California, Santa Cruz, published this week in the online early edition of Proceedings of the National Academy of Sciences (PNAS).

The idea that morphs within the same population could eventually evolve into separate species is not a particularly new one. But a study by researchers Barry Sinervo, a professor of ecology and evolutionary biology at UCSC, and Ammon Corl, who led the new study as a graduate student at UCSC but is now a postdoctoral researcher at Uppsala University in Sweden, about side-blotched lizards finds new evidence to support the theory.

So where does rock-paper-scissors come into play? The side-blotched lizard has three morphs differing in color (orange-, blue-, and yellow-throated) and mating behavior. Previous work by Sinervo and his colleagues showed that competition among male side-blotched lizards resembles the game, in that each mating strategy beats and is beaten by one other strategy. Orange-throated males (we’ll call them rock) with their high testosterone and body mass can take territory from blue-throated males (scissors), allowing them to control large territories containing many females. Rock crushes scissors. Yellow-throated males (paper) mimic female behavior and coloration to sneak onto the large territories of orange males to mate with females. Paper covers rock. And blue-throated males cooperate with each other to defend territories and closely guard females, beating the yellow-throated males’ covert strategy. Scissors cut paper.

Using DNA analysis and studying numerous populations of the lizards over time, the researchers found that all three color morphs existed millions of years ago and have persisted in many populations, but some branches of the lizard family tree lost some of the color types over time. Sometimes the rock-paper-scissors game cycles on indefinitely while in other places it “collapses on one or two strategies and begins to create new species,” says Sinervo in a February UCSC press release.

Also in the press release, Corl uses the rock-paper-scissors analogy to explain other new evidence found in the study. “Imagine the three lizard morphs playing rock-paper-scissors,” Corl explains. "They have very specific adaptations for fighting one another. Now imagine that some morphs are lost, leaving a population of all rock morphs. Their adaptations for fighting the paper and scissors morphs are no longer useful. Therefore, rapid evolutionary change is expected in a population of rock morphs as they adapt to a new game in which they only fight other rock morphs."

The study showed clear evidence of very rapid evolution of body size when morphs are lost from a population. “Such rapid evolution could eventually cause populations to evolve into distinct species. We are the first group to provide a statistical test of this hypothesis,” Corl say in the press release.

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