Variation is key to evolution. Differences between individuals within a population, between populations of the same species, and between closely related species represent successive stages in the formation of new species. Ph.D. student Elizabeth Bastiaans and Dr. Barry Sinervo will study individual, population and between species variation in the mesquite lizard species complex, a Mexican lizard whose evolutionary relationships are not yet fully understood. The mesquite lizard inhabits the mountains of central and northern Mexico, an environment that is changing rapidly due to both climate change and increasing urbanization. Ms. Bastiaans and Dr. Sinervo?s work with these lizards will help illuminate how new species form and adapt to the environment. Within populations, the lizards vary in the color of brightly pigmented patches used in mating and aggressive interactions. Between populations and closely related species, the lizards vary in coloration, habitat use, and genetic characteristics. The researchers will use field surveys, behavioral experiments, and molecular tools to address how ecology, behavior, and genetics contribute to divergence between populations in the mesquite lizard. The results of the research will shed light on which populations are most vulnerable to decline or extinction due to climate change and/or habitat loss. This work fosters collaboration between the United States and Mexico on problems that show no respect for borders, and it provides opportunities to train and educate high school and college students from both nations.
Many animals use brightly colored patches of skin to communicate with other individuals of the same or opposite sex during breeding. This phenomenon has been especially well studied in lizards. In many lizard species, different individuals within the same population have different colors. This type of variation is called a polymorphism, and the different colors are called color morphs. In several species of color polymorphic lizards, different color morphs also show differences in behavior. Recently, researchers have started to find that, in color polymorphic species, there is also often variation among different populations in how many morphs are present, which morphs are present, and how common particular morphs are. These types of differences could make it less likely that individuals from different populations interbreed, which can contribute to the formation of new species. We studied the mesquite lizard (Sceloporus grammicus), which lives primarily in mountain habitats in Mexico. Other researchers thought that the mesquite lizard might represent a case of new species formation in progress, because there are unusual genetic differences among populations. Studies of how different populations were related to one another genetically had yielded some confusing results, however. There was anecdotal evidence that both male and female mesquite lizards had throat color polymorphisms, but no one had yet done a formal study of them. We thought studying differences among populations in throat color might help explain some of the confusing results regarding which populations interbreed with which other populations. Studying differences among individuals within populations could also help us understand how color conveys information about behavior in animals. We sampled 22 populations and found that there are two types of throat color polymorphism in the mesquite lizard. In some populations, males can have orange, yellow, or blue on their throats; whereas females can have orange, yellow, or white. These colors also occur in several related lizard species. In other populations of the mesquite lizard, however, both males and females can have orange, yellow, or white on their throats. We never found blue and white males together in any population. We used DNA to make a phylogeny, or "family tree," of the populations we sampled, which showed that the change from blue males to white males has occurred at least twice during the evolution of the mesquite lizard. We focused on two particular populations, one with blue males and one with white males, to study whether males of different throat colors behaved differently, as had been found for many other species with color polymorphisms. We found that males of different color morphs differed in aggression in both populations. Even though orange and yellow color morphs occurred in both populations, the colors didn't mean the same thing about a male's aggression in the two different populations. We also found that blue and white males were behavioral opposites: blue males were more aggressive than other males in their population, whereas white males were less aggressive than other males in their population. We used the same two populations as we used for the male aggression study to ask how the presence of blue males in one population and white males in another population affects a female's ability to determine whether a male is from the same population as she is or not. We found that females more strongly rejected the courtship attempts of "foreign" males of unfamiliar morphs (blue or white) than those of "foreign" males of familiar morphs (orange or yellow). Intellectual Merit: Our research is one of very few studies to indicate that, even within the same species, a particular color may not always mean the same thing about an individual animal's behavior. The results of our female choice experiment may help explain why it has been so difficult to study how populations of the mesquite lizard and other polymorphic species are related to each other. The likelihood that interbreeding occurs between different populations may be affected not only by genetic differences between them, but also by the relative frequencies of familiar vs. unfamiliar color morphs where the populations come into contact. In general, our research indicates that variation among populations in taxa with color polymorphisms within populations likely plays an important role in their evolution. Broader Impacts: The mesquite lizard lives in high elevation habitats that are changing rapidly, due to climate change and urbanization. Our phylogeny helps identify groups of populations that differ from nearby populations both genetically and in their sexual signals. Knowing these populations are unique can help conservationists make better arguments for conserving those habitats. It also appears likely that similar color polymorphisms in species related to the mesquite lizard may be used as rough indicators of how much interbreeding occurs among different populations. If two nearby populations have different numbers, frequencies, or colors; they are probably genetically different and should be managed separately.
