Previous research has shown 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.
For the current study, scientists observed lizard populations from California, Texas and Washington State and found the three color morphs in many places, but not everywhere. Some populations were missing some of the color morphs. For example, populations in the northwest only have orange-throated lizards, while only orange- and blue-throated morphs are found on Anacapa Island in the Channel Islands. In the field, the researchers captured lizards to collect tissue samples for DNA analysis and then released them back into the wild. In the lab, they used the tissue samples to get DNA sequences from all of the lizard populations in the study.
Photo by Ammon Corl
"Based on these sequences, we reconstructed the 'family tree' of the lizard populations and figured out which populations were more closely related to one another. This let us figure out how the mating strategies evolved," said Ammon Corl, a postdoctoral researcher at Uppsala University in Sweden.
The results showed that all three color morphs existed millions of years ago and have persisted since then in many populations. Over time, however, some branches of the lizard family tree lost some of the color types.
"What was particularly interesting was the pattern in how color morphs were lost," Corl said. "Any time there was a loss, the yellow type--the sneaky males that mimic females--was the first to go. Thus, the rock-paper-scissors game can break down on an evolutionary timescale. Something about the game must change so that, for instance, both the rock and scissors strategies are able to beat paper."
"Ammon's research indicates that the game has been cycling for millions of years at some sites, and yet at other sites it collapses on one or two strategies and begins to create new species. It is simply mind-boggling to think about deep time and these evolutionary cycles," said Barry Sinervo, a professor of ecology and evolutionary biology at UCSC.
Many aspects of the evolutionary history of these lizards are consistent with the theory that morphs can be involved in speciation. 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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