Female swallowtail butterflies do something a lot of butterflies do to survive: they mimic wing patterns, shapes and colors of other species that are toxic to predators. Some – but not all – swallowtail species have evolved several different forms of this trait. But what kind of genetic changes led to these various disguises, and why would some species maintain an undisguised form when mimicry provides an obvious evolutionary advantage?
In a new study published this week in Nature Communications, scientists from the University of Chicago analyze genetic data from a group of swallowtail species to find out when and how mimicry first evolved, and what has been driving those changes since then. It’s a story that started around two million years ago, but instead of steady, progressive changes, one chance genetic switch helped create the first swallowtail mimics. And it has stuck around ever since.
If it is not progressive change, it is not Darwinism
For a long time, scientists thought that butterfly mimicry was controlled by “supergenes,” groups of several tightly linked genes that were always inherited as a group. In a 2014 study, Kronforst and his colleagues showed what appears to be a supergene is actually a single gene called doublesex that controls the different color patterns and shapes we see in female swallowtails.
It’s not at all clear whether double sex is an advantage to the butterfly:
The researchers also looked at what maintained polymorphism over time. One cause could be sexual selection, that males prefer certain female color patterns over another. Previous research on mating behavior doesn’t back up that idea though. Another possibility is “crypsis,” or the idea that undisguised females blend into their natural surroundings better than the mimics. Kronforst and the team tested that hypothesis by comparing mimetic and non-mimetic females against a green forest background using models for predator (i.e. bird) vision. The non-mimetic, undisguised females actually don’t blend in to the background any more than mimics, so this idea is out too
Those two findings, combined with the genomic sequence data, led the researchers to start thinking about another intriguing possibility. It could be that the genetic changes that led to mimicry in the first place also built in some long-term disadvantages. When the original doublesex gene inverted, it probably carried a bunch of other unrelated genetic material with it. Since the flipped doublesex gene can’t be recombined with its original version, the extra stuff has “hitchhiked” ever since—and it could have consequences. In fact, some research shows that female mimics don’t live as long as standard ones.
“We think a bunch of differences were accidentally captured when one copy of the gene flipped and became the mimetic copy. Because a lot of those changes are functional, they could be detrimental to health,” Kronforst said.
“The idea is that you have this hardwired disadvantage to mimicry. The standard females don’t have the protection of mimicry, but they also don’t have this inherent genetic cost and these two things offset one another” he said. More.
It’s all fascinating. The big disadvantage of Darwinthink, of course, is that it can explain everything and anything and therefore, eventually, nothing. A theory that could make highly specific predictions might be more useful. One learns more by confuting specifics than by cming up with ad hoc explanations.
See also: Convergent evolution: Speciation in butterflies an unusually tough mess
Evolutionary convergence of butterflies
The traditional “supergene” view:
The caterpillars have their own tricks: