[Featured image: An Anolis evermanni lizard, photo courtesy Edmund D. Brodie III.]
A new study from Virginia Tech takes on the decades-old battle of which has more impact on evolution: genetic variation or natural selection.
In a study published in the latest issue of Evolution Letters, Virginia Tech researcher Joel McGlothlin has found that genetic variation can leave a much longer-lasting stamp on evolutionary patterns than was previously thought. Started when McGlothlin was a post-doctoral researcher at the University of Virginia, the study focuses on Anolis lizards, which McGlothlin and other scientists say are “icons” of adaptive radiation, an evolutionary pattern involving the origin of group of related species that differ in appearance and ecological role.
“Different anoles species have evolved different traits that allow them to live in different habitats such as in treetops or on tree trunks,” said McGlothlin, an associate professor in the Department of Biological Sciences, part of the Virginia Tech College of Science. “During the past 40 million years or so, species with body types fitting them into these habitats have evolved several times across different islands in the Caribbean. This suggests that natural selection has had similar effects on evolution under similar conditions.”
However, scientists know that natural selection doesn’t always push a species in the optimal direction, McGlothlin said. Because natural selection works with existing genetic variation, evolution can be “constrained” by genetics. For example, if some traits are not as heritable as others, they may evolve more slowly. Also, when traits are correlated with each other — such as arm length and leg length — it may be more difficult for them to evolve on their own, he added. Although these constraints are important over a few generations, whether they are important over millions of years of evolution is more controversial.
McGlothlin and his team sought to disentangle the roles that natural selection and genetic constraints played in the evolution of body shape among anoles. “What we found was the species become differentiated from each other in ways predicted not only by their habitat, but also by patterns of genetic variation,” McGlothlin said. “Traits that were more genetically variable showed greater evolutionary changes across species. We were really surprised that we still saw this pattern when looking across 40 million years of evolution.”
In the study, McGlothlin and his team — which included Edmund “Butch” Brodie III, a professor at the University of Virginia and McGlothlin’s former postdoctoral mentor, and Jonathan Losos, a professor of biology at Washington University in St. Louis, Missouri — measured patterns of genetic variation for body-shape traits in seven different species of anoles and compared it to how traits evolved across species. The team also included several undergraduate students from several universities.
They collected adult lizards from Puerto Rico, Jamaica, and the Bahamas and bred them in the lab to produce thousands of offspring. Measuring traits, such as head shape and limb length, in these offspring allowed the researchers to measure how much trait variation was due to heritable differences that could be passed down from parent to offspring.
The team found another surprising result: The relationship between genetic variation and evolution was maintained even though the genetic variation they measured also changed across evolutionary time. Their analysis suggests that that genetic variation isn’t just passive material for natural selection. Instead, it seems to co-evolve with the traits themselves, perhaps changing in response to selection.
“When we began this study, we thought we might be able to provide strong evidence favoring either selection or constraint, but instead, we may have demonstrated just how difficult they are to separate,” McGlothlin wrote in a blog post for Evolution Letters. “At least in anoles, constraint shapes the evolutionary response to selection, but also evolves in response to selection in such a way to keep the two entwined. Perhaps it’s this never-ending creative dance that makes evolution so interesting in the first place.”
The study of these lizards can help scientists understand the evolution of other species, said McGlothlin, who is an affiliated member of the Fralin Life Science Institute’s Global Change Center. “Our results are pretty general, and I wouldn’t be surprised if we saw similar patterns if we looked genetic variation in humans and our closest relatives,” he added.
McGlothlin is continuing to research the role of genetic variation in evolution. “Now, we are asking some similar questions using a single species, the brown anole,” McGlothlin added. “In that species, males and females are really different, and we’re trying to apply what we’ve learned about the evolution of different species to understand how males and females evolve to become different in appearance.”