Hurricane-Ravaged Lizard Populations Provide Rare Glimpse at Evolution in Action

We’re often told to think of the lack of genetic variation in small populations as a bad thing. Think of problems like hip dysplasia in dogs due to very narrow breeding regimens that limit genetic diversity over time. But small populations have also long been posited as a driver in evolution because mutations are able to more quickly spread.

However, it’s a phenomenon that hasn’t been studied easily in the wild until now: After a 2004 hurricane obliterated a Caribbean lizard population, UC Davis researchers were able to study how the limited population left changed over time.

The process in question is called the founder effect, first posited by German evolutionary biologist Ernst Mayr in the early 1940s. The founder effect essentially argues that when a limited number of individuals establish a population separate from a larger group, a loss of genetic variation leads to the new population being distinct from the larger one it split from. Over time, those different traits will persist in the founder population. In a sense, it’s a riff on island biogeography theory, like in the case of the finches with specialized beaks studied by Darwin.

The issue is in the details. Some biologists argue that the founder effect is a critical factor in the evolution of a species and diversification. On the other hand, critics argue that the spread of random mutations – a key aspect of the founder effect – throughout a population are outweighed by the more-target forces of natural selection. It’s an argument that’s persisted because it’s very hard to study the effect in the wild.

“Founder effects are very hard to study,” Thomas Schoener, a UC Davis professor and co-author of the study, said. “One must be in exactly the right place at the right time to observe the founder event — and then fortunate enough to be able to follow a population through time.”
The UC Davis team, along with researchers from Harvard and Duke, received the chance to put the theory to the test for the first time in the wild in September 2004, when Hurricane Frances submerged a number of small islands in the Bahamas. Prior to the hurricane, these islands were known to harbor the brown anole. After the storm, the team found seven of these islands that no longer had a single lizard on them.

The following May, the researchers re-populated on those seven islands by placing a single male-female pair of anoles on each, selected from wild populations on other, larger islands. Over four years, the team compared individuals from the source island, the experimental islands, and a dozen other islands that served as a control.

The results, to be published in Science on February 17, suggest that both sides of the founder effect argument have a hand in evolutionary processes. All the lizards involved adapted to the specific environments of their individual islands, while at the same time keeping some of the traits of their parent founders.

"Our study is an entirely unique approach to a question of longstanding importance for evolutionary biology regarding the founder effect: Will it persist in the face of the strong selection that would often exist in the colonized environment?" Schoener said. “The answer we found is that founder effects can leave a persistent signal as generations replace one another over time, even as populations adapt to new conditions. Our study of these fundamental evolutionary principles affects our general understanding of how the biological world works.”

One characteristic look at was the lizards’ limb lengths. It’s been shown that limb length correlates with the average diameter of vegetation on an island. Each founder island had smaller vegetation than the larger source island, and thus the length of the lizard limbs on those seven islands decreased over time due to natural selection.

At the same time, the founder populations kept the traits of their parents. So while the limb length of all the lizards decreased in founder populations, populations that had parents with comparatively longer limbs still had longer limbs at the end of the study.

“Natural selection drives them all down, while the founder effect keeps the order the same,” Schoener said. “So they’re both right, in a sense.”
If natural selection had completely overpowered the founder effect, all of the lizards’ limbs would have decreased to become a similar length (vegetation across the seven tests sites was similarly sized). Because limb length decreased more or less equally across the board, the study suggests that both the founder effect and strict processes of natural selection have a role in the evolution of populations.