Evolutionary game of rock-paper-scissors may lead to new species

New research on lizards supports an old idea about how species can originate. Morphologically distinct types are often found within species, and biologists have speculated that these "morphs" could be the raw material for speciation. What were once different types of individuals within the same population could eventually evolve into separate species.

A new study conducted by researchers at the University of California, Santa Cruz, supports this idea. The study documents the disappearance of certain morphs of the side-blotched lizard in some populations. The researchers reported their findings in a paper published this week in the online early edition of Proceedings of the National Academy of Sciences (PNAS).

The side-blotched lizard, Uta stansburiana, has three morphs differing in color and mating behavior. Barry Sinervo, a professor of ecology and evolutionary biology at UCSC, has studied a population of side-blotched lizards near Los Baños, Calif., for over 20 years. Ammon Corl, now a postdoctoral researcher at Uppsala University in Sweden, led the new study as a graduate student at UCSC and is first author of the paper.

Previous work by Sinervo and his colleagues showed that competition among male side-blotched lizards takes the form of a rock-paper-scissors game in which each mating strategy beats and is beaten by one other strategy. Males with orange throats can take territory from blue-throated males because they have more testosterone and body mass. As a result, orange males control large territories containing many females. Blue-throated males cooperate with each other to defend territories and closely guard females, so they are able to beat the sneaking strategy of yellow-throated males. Yellow-throated males are not territorial, but mimic female behavior and coloration to sneak onto the large territories of orange males to mate with females.

"My goal when starting my Ph.D. thesis research was to understand how this fascinating mating system evolved," Corl said. "We studied lizard populations from California to Texas and from Washington State down to Baja California Sur in Mexico." ...

Many aspects of the evolutionary history of these lizards are consistent with the theory that morphs can be involved in speciation, Corl said. Evolutionary theory predicts that new species could arise from particular morphs originally found in a population containing multiple morphs. Side-blotched lizards started off with three color morphs. If just one or two types occur in a population, they look just like the original morphs.

The theory was also supported by patterns in the formation of subspecies, which are the precursors to new species. Two subspecies of side-blotched lizard that originated from populations with three morphs now have only a single color morph. Thus, populations that lose morphs are not transitory, but can persist and eventually become a different species.

The study also found evidence to support the hypothesis that rapid evolutionary change occurs when particular morphs are lost from the system. "Imagine the three lizard morphs playing rock-paper-scissors," Corl explained. "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 said.

The idea of morphs being involved in speciation is an old one. Charles Darwin conducted experiments with different reproductive morphs in flowers to try to gain insight into the process of speciation. However, the new paper by Corl and colleagues is one of the first studies to use modern techniques to tackle the problem of morphs and speciation. ...

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