Long-term observations of lizards challenge conventional wisdom about natural selection, showing that species can evolve while maintaining a consistent appearance. Many species change little over long periods of time. Biologists often use the same explanation for why this happens: natural selection favors individuals with milder traits. Individuals with more extreme traits, such as longer limbs, are usually at a disadvantage, while more moderate or average individuals are more likely to survive and reproduce, passing on their common traits.
New research provides new insights into evolutionary stalling by studying how lizards survive in their natural habitat. Contrary to conventional wisdom, this study found that natural selection maintains species-average traits infrequently. Instead, it revealed that traits that favor survival varied from year to year, but overall, species appearance remained largely unchanged over time.
But new research from Washington University in St. Louis and the Georgia Institute of Technology provides a more comprehensive explanation of evolution among coexisting species. By directly measuring lizards' long-term survival in the wild, scientists found that coexisting species each occupy a unique "adaptive peak," which is best understood as part of the overall community's "adaptive surface," or landscape.
The research, led by James Stroud of the Georgia Institute of Technology, was published this week in the Proceedings of the National Academy of Sciences.
Take high-resolution photos of lizard feet to measure the size of adhesive subdigital pads. Image source: Day'sEdgeProd
"If a species adapts to its environment and the environment doesn't change, you wouldn't expect the species to adapt," said Jonathan Losos, professor of biology in the College of Arts and Sciences and the William H. "When scientists went to study natural selection, however, they found little evidence of this stabilizing selection, so we set out to study natural selection in an organism we're very familiar with: the Anolis lizard, measuring selection over several years to try to understand what's going on."
Stroud, then a postdoctoral fellow in Losos' lab at the University of Washington, discovered four different species of burrowing lizards living together on an island in a lake at the Fairchild Tropical Botanic Gardens near Miami.
He captured thousands of individual lizards on the island, tagged them individually and measured their body proportions. Stroud then recaptured all the lizards on the island every six months for two and a half years, enough time for them to produce two or three generations of lizards.
James Stroud uses a small lasso attached to a fishing rod to catch the lizards. Image source: Day'sEdgeProds
The new lizards are apparently children of the island. If a lizard disappears from his census list, Stroud can be sure it's dead because the surrounding lake is filled with predatory fish. By determining which lizards survive from one year to the next, researchers can assess whether survival rates are related to the physical characteristics they have been measuring, such as leg length.
"What's special about this study is that we simultaneously measured natural selection in four coexisting species, something that very few people have been able to do," Losos said. "Coincidentally, at the same time as our paper was published, another research group published a similar study on Darwin's famous Galapagos finches."
In Florida lizards, Losos and Stroud found that stable forms of natural selection—the maintenance of the same average characteristics across a species—are extremely rare. In fact, natural selection changes dramatically over time. In some years, lizards with longer legs survive better, and in other years, lizards with shorter legs survive better. Other times, there's no discernible pattern at all.
Stroud said: "The most fascinating result is that natural selection varies very much across time periods. We often see the direction of selection completely reversed from one year to the next. However, when combined into a long-term pattern, all these changes effectively cancel out: species remain very similar throughout the time period."
Scientists don't yet fully understand how evolution works at the community level. Long-term studies like this are rare because they require a lot of work and time.
"Evolution can and does happen - it's an ongoing process, but that doesn't necessarily mean things are constantly changing over the long term," Stroud said. "Now we know that evolution is still happening even when animals look like they've stayed the same."