The evolution of long-winged butterflies in Costa Rica may prove the classic Hollywood advice: Fake it until you make it.
A relatively small change in a part of the genome that changes wing coloration and pattern kick-starts the divergence of a species, and the butterflies' promiscuous habit of interbreeding may play a larger role in that process than previously thought, according to a study published online Thursday in the journal Cell Reports.
The first-time genome comparison of five key species of long-winged Heliconius butterflies sheds new light on the role of gene flow, or hybridization, between nascent species that remain in contact with each other, and helps bring the process of natural selection into sharper focus.
"Historically, people think of speciation as being an accident of being separated for a long time," said Marcus R. Kronforst, a University of Chicago evolutionary biologist and lead author of the paper. "But here we've got natural selection to mimic different things that's really causing speciation."
The results are "an example of the extraordinary power evolutionary geneticists now control to investigate the effects of population processes such as gene flow at every single site in the whole genome," said James Mallet, an evolutionary biologist at Harvard University who was not involved in the study.
The Heliconius genus offers an opportune test of the mechanism of evolution – it has radiated into 45 known butterfly species, including the Costa Rican postman, relatively recently. And its genetic thievery and promiscuous habits helped geneticists solve the puzzle.
While complex interbreeding -- including between hybrids and ancestor species -- would make a family reunion exasperating, it actually makes the job of genetic analysis of the butterfly genomes simpler in an important way: Hybridization tends to homogenize background genetic variance in areas that are not under selective pressure. The result: The areas under selective pressure are easier to distinguish from changes that may have occurred long after the species diverged. Signal emerges from noise.
For the Heliconius species in the study, the zone under pressure from natural selection turns out to be a very narrow area that centers around color and enables a type of mimicry (Mullerian mimicry) by which one toxic species imitates another, creating a shared survival advantage by baffling predators into avoiding a range of wing patterns.
Previous research has shown that hybridization contributed to the variation in coding for wing patterns that could be subjected to natural selection. But as patterns changed, so did mating choices, eventually thwarting gene flow between the nascent species.
"When they shift their color patterns, that generates all sorts of reproductive isolation because the individuals with different color patterns don't recognize each other as mates anymore, so they stop mating with one another," Kronforst said.
Comparison of species that parted ways recently with those that diverged long ago also revealed an exponential growth from about 165,000 DNA base pair sequence variations to a whopping 19-33 million over the period of about a million years, the study found. That explosion also was largely driven by the rise of new areas of genome variation, and not merely by expansion of the original "island" of variation, the study showed.
"The vast majority of the growth is brand new regions that pop up later," Kronforst said.