Butterflies Lend Insights About Speciation

Early signs of divergent evolution in Heliconius butterflies in Ecuador may reveal a missing link to understanding how species form, according to a recent study by Harvard researchers.

The research, published today in the journal “Science,” indicates that evolution due to factors like advantageous mimicry may influence changes in mate preference that further propel speciation.

“In nature, you can find things that are clearly the same or clearly different species,” said research fellow Marcus R. Kronforst, an author on the study affiliated with the Faculty of Arts and Sciences Center for Systems Biology.

“In Ecuador we found something in-between, which provided a big window into understanding how species form.”

The recent study in Western Ecuador was prompted by a 2002 study in Costa Rica, which focused on two species of geographically separated butterflies with a common ancestor. Although these white– and yellow–winged species had once been the same species, members of each preferred mating with butterflies of the same species—an example of what is known as assortative mating preference.

To further explore the connection between speciation and mating preference, Kronforst and four other researchers embarked for Western Ecuador in May 2008.

For two months, Harvard biology post-doctorates Nicola L. Chamberlain and Ryan I. Hill, both authors on the study, stood in an enormous cage of Heliconius butterflies, ready to swoop in and grab males as they fluttered their wings above a female in an attempt to mate.

Chamberlain said the fieldwork was a combination of extremes, involving exciting mountain—bike excursions to collect butterflies followed by hours of sitting and waiting for them to mate. “It was sometimes frustrating when you felt like you were giving it your all but the butterflies were not performing,” Hill added.

Kronforst said that the butterflies in Ecuador were diverging into two groups because each subgroup was evolving to look more like a different species of poisonous butterfly, which conferred an advantage since predators learned more quickly to avoid them.

This mimicry caused some of the population to have yellow wings and others white wings, but the Heliconius group in Ecuador was still genetically identical in every other way, unlike in Costa Rica.

The team found that assortative mating preference in Ecuador was observed some of the time, while in Costa Rica it was observed nearly all the time.

“We think that what we were seeing in Ecuador is an earlier stage in the process of speciation that occurred in Costa Rica,” Kronforst said.

The researchers undertook additional genomics lab work and research into related studies to prove that the population in Ecuador had not yet diverged into two separate species, setting it apart from around 20 other systems that had been studied in the past.

Natural sciences professor Hopi E. Hoekstra, who studies speciation in mice, is excited about the genetic implications of Kronforst’s research.

“Mimicry causing divergence is one thing, but if you have mate choice based on color differences, you’re really setting the stage for finding genes that contribute to reproductive isolation,” she said.