Writing to Charles Darwin in 1861, naturalist Henry Walter Bates described brightly colored Heliconius butterflies of the Amazon as “a glimpse into the laboratory where Nature manufactures her new species.” More than 160 years later, an international research team including Harvard scientists also focused on this genus of butterflies to document the evolution of a new species.

In a paper published April 17 in Nature, the team found that hybrids between two Heliconius species of butterflies produced a new species genetically distinct from both parent species and their earlier forebears.

Researchers used whole-genome sequencing to show that a hybridization event some 180,000 years ago between Heliconius melpomene and the ancestor of today’s Heliconius pardalinus produced a third hybrid species, Heliconius elevatus. Although descended from hybrids, H. elevatus is a distinct butterfly species with individual traits — including its caterpillar’s host plant and the adult’s male sex pheromones, color pattern, wing shape, flight, and mate choice. All three species now fly together across a vast area of the Amazon rainforest.

“Historically hybridization was thought of as a bad thing that was not particularly important when it came to evolution,” said Neil Rosser, an associate of entomology at Harvard’s Museum of Comparative Zoology who co-authored the study and handled its genetic mapping with Harvard postdoctoral fellow Fernando Seixas. “But what genomic data have shown is that actually hybridization among species is widespread.”

The findings may alter how we view species. “A lot of species are not intact units,” said Rosser. “They’re quite leaky, and they’re exchanging genetic material.”

Historically, hybridization has been thought to inhibit the creation of new species. Yet in this case, the researchers say, hybridization is driving the evolution of a new species. “The species that are evolving are constantly exchanging genes, and the consequence of this is that it can actually trigger the evolution of completely new lineages,” said Rosser.

“Normally, species are thought to be reproductively isolated,” added co-author James Mallet, professor of organismic and evolutionary biology at Harvard. “They can’t produce hybrids that are reproductively fertile.” While there is now evidence of hybridization between species, what was difficult to confirm was that this hybridization is, in some way, involved in speciation. As Mallet put it: “The question is: How can you collapse two species together and get a third species out of that collapse?”

The new research provides a next step in understanding how hybridization and speciation work. “Over the last 10 or 15 years, there’s been a paradigm shift in terms of the importance of hybridization and evolution,” Rosser said.

This research has the potential to play a role in the current biodiversity crisis. Understanding something as basic as what we mean by a species is important for saving species and for conservation, particularly in the Amazon, Mallet said. In addition, such work may prove useful in understanding carriers of disease. Multiple species of mosquito, for example, can carry malaria. Although these mosquitos are closely related, “almost nothing is known about how they interact, and whether they hybridize with each other,” he noted.

In addition to co-lead author Kanchon Dasmahapatra of the University of York in England, the international crew of researchers included Lucie M. Queste, Bruna Cama, Ronald Mori-Pezo, Dmytro Kryvokhyzha, Michaela Nelson, Rachel Waite-Hudson, Matt Goringe, Mauro Costa, Marianne Elias, Clarisse Mendes Eleres de Figueiredo, André Victor Lucci Freitas, Mathieu Joron, Krzysztof Kozak, Gerardo Lamas, Ananda R.P. Martins, W. Owen McMillan, Jonathan Ready, Nicol Rueda-Muñoz, Camilo Salazar, Patricio Salazar, Stefan Schulz, Leila T. Shirai, and Karina L. Silva-Brandão.