Life may “find a way,” but how living things evolve is not a neat and orderly process. Instead of a tidy family tree with straight lines added for each new generation, the birth of a new species is much more tangled in reality. New research into one butterfly genus found in the Amazon shows just how entangled those evolutionary lines may be. Hybrids between some species can produce new butterfly species that are genetically distinct from both parent species and their earlier ancestors. The findings are described in a study published April 17 in the journal Nature

A third hybrid

In the study, the team focused on the brightly colored Heliconius genus of butterflies found in Central and South America. They are a common model for studying how butterfly wing patterns evolved due to the wide variety of wings within the group. In an 1861 letter to Charles Darwin, naturalist Henry Walter Bates referred to the Heliconius butterflies found in the Amazon as “a glimpse into the laboratory where Nature manufactures her new species.”

a speciation tree diagram that shows how three butterfly species emerged
A family tree of the parent butterfly species, showing the main hybridization events. CREDIT: Neil Rosser

For a deeper look into Heliconius’ evolution, the team on this new study harnessed the power of whole-genome sequencing. All living organisms have DNA that is made of four nucleotide bases–adenine, thymine, cytosine, and guanine. If you know the sequence of bases, you can identify the organism’s unique DNA fingerprint called a pattern. Sequencing determines these patterns and whole genome sequencing in a lab can determine the orders of these bases in one process.  

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The whole-genome sequencing indicated that a hybridization event occurred about 180,000 years ago between Heliconius melpomene and the ancestor of today’s Heliconius pardalinus butterflies. This event produced a third hybrid species called Heliconius elevatus. While it is descended from hybrids, H. elevatus is also a distinct butterfly species and has its own individual traits. These include color pattern, wing shape, flight characteristics, how they choose mates, and more. All three of these distinct species now fly together across a wide area of Amazon and indicate more evidence that hybrids are not always sterile as sometimes previously thought. 

“Historically, hybridization was thought of as a bad thing that was not particularly important when it came to evolution,” study co-author and Harvard University biologist Neil Rosser said in a statement. “But what genomic data have shown is that, actually, hybridization among species is widespread. Over the last 10 or 15 years, there’s been a paradigm shift in terms of the importance of hybridization and evolution.”

An evolutionary surprise

According to the team, this may alter how we view species and speciation. Scientists had generally believed that hybridization inhibited the generation of new species. Hybrid organisms are often born unhealthy or sterile and can’t reproduce, particularly when they are born with two different sex chromosomes. Most species are not perfectly intact tight units, but instead exchange a lot of DNA and can be considered “quite leaky.” The species that are evolving are actually exchanging genes constantly and it can trigger the evolution of new lineages. 

“Normally, species are thought to be reproductively isolated. They can’t produce hybrids that are reproductively fertile,” study co-author and Harvard University biologist James Mallet said in a statement

This is a different case for Heliconius  butterflies. They show that hybridization is not only occurring, but has driven the evolution of a new species in itself. While there is now evidence of hybridization between species, confirming if hybridization is involved in speciation has been difficult. 

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“The question is: How can you collapse two species together and get a third species out of that collapse?” said Mallet.

This new research provides scientists with a next step in understanding how hybridization and speciation work in evolution. It could also help play a role in the planet’s biodiversity crisis, since fully understanding the question of what we really mean by “species” on a genetic level is important for conservation. It may also help in understanding the carriers of certain diseases. Multiple species of mosquitoes carry malaria, and while they are closely related, we still do not know how they interact or create new hybrids the way Heliconius butterflies do. 

As with evolution itself, this area of study will only continue to untangle as biologists learn more about what really makes a species a species.