There’s no guarantee evolution will bestow the best version of every trait. The benefits of one trait may impose a cost when an unfavorable trait, correlated genetically, comes along for the ride. In a recent study on wood frogs (Rana sylvatica) published in The American Naturalist, researchers found evidence suggesting that metamorphosis might reduce such cases of genomic conflict.
The findings may explain why metamorphosis, a risky and physiologically expensive process, persists in nature. “It’s adaptive,” says paper coauthor and evolutionary biologist Debora Goedert, a graduate student at Dartmouth College in Hanover, NH, and a fellow of Brazil’s Science Without Borders program. “And the reason why it’s adaptive is because it allows organisms to separate these two life stages in such a way where they can be optimum in both life stages and not carry some sort of specialization tradeoff where becoming a really good tadpole now means you are a terrible frog, or vice versa.”
The wood frog must adapt to two wildly different habitats: ephemeral pools for its tadpole stage, and the forest floor for its frog stage. Preliminary research had suggested that metamorphosis might effectively compartmentalize genetically correlated traits, allowing natural selection to act only on the trait that’s needed during a particular life stage—such as tails in tadpoles—rather than simultaneously enhancing a positive trait and an unrelated but genetically correlated disadvantageous trait.
The recent study combines quantitative genetic evidence with estimates of natural selection, bolstering the hypothesis that metamorphosis may be adaptive. Breeding studies in the lab enabled a look at genetic correlations between traits, such as tadpole tail length, frog leg length, and head size in both life stages. To examine how natural selection acted on these traits, the researchers built simulated ponds in the forest. There, they measured predation on tadpoles by beetle larvae with giant mandibles, on juvenile frogs by garter snakes, and on adult frogs by kestrels and small mammals.
The researchers not only identified genetically correlated traits undergoing conflicting selection within and between the wood frog’s two life stages, but also determined that metamorphosis alters the degree of correlation of the traits throughout the frog’s lifespan. They found that the degree of genomic conflict was lower between life stages than within for both the tadpole and frog, suggesting that metamorphosis lowers the conflict between antagonistic traits.
“A strength of this article is that they’re trying to cast genomic conflict in terms of descriptions at the trait level,” says Stevan J. Arnold, an evolutionary biologist and professor emeritus at Oregon State University in Corvallis who was not involved in the research. “I think this is a big triumph, to have done a study that quantifies both the nature of selection and the nature of genetic correlation between traits.” The work “helps us understand how and why metamorphosis evolves,” Arnold adds.
Whether or not these observations represent “genomic conflict,” however, is a matter of some debate. Traditionally, researchers have defined it in terms of evolutionary conflict between identifiable genes. But so little is known about the frog genome relative to other organisms that the researchers had to broaden the definition to include traits that are likely controlled by multiple genes. Nonetheless, they say, the breeding studies point to correlated genetic elements underlying the traits—they’re just not able to identify them yet. “It’s an early stab at establishing genomic conflict,” says Goedert. She and her advisor, coauthor Ryan Calsbeek, also an evolutionary biologist at Dartmouth, acknowledge that they have not yet established a definitive causal link between metamorphosis and the decoupling of traits embroiled in genomic conflict.
The field is trying now to “figure out how to grapple with testing both this idea of antagonistic selection and its consequences,” Arnold says. “For a full testing of the theory there’s a need for a more detailed enunciation of the underlying model. What’s the bigger picture? That’s an agenda probably for the next ten years.”