Putting One and One Together

Butterfly blend.
Two different butterfly species apparently mated to create a third in the South American Andes.

Mauricio Linares, Uniandes, Colombia

Like merging two companies to create a joined venture, scientists have bred two colorful butterfly species to form a third, apparently duplicating a feat nature has already accomplished. This speciation process, known as hybridization, may play a larger role in evolution than is currently believed, the researchers propose.

It's well-accepted that a new species will arise when a subset of an existing species breaks off and accumulates too many genetic mutations to breed with members of the original species. But many researchers have uncovered hints that two species can also merge their gene pools to come up with a third (ScienceNOW July 27, 2005). For this to happen in nature, however, the newly formed species would need to be somehow separated from the other two parental species; otherwise, it would simply get sucked back into the old populations.

A team led by evolutionary biologist Jesús Mavárez of the Smithsonian Tropical Research Institute in Panama studied that problem in three similar-looking butterfly species living in parts of Central and South America. Judged by its color pattern, a species called Heliconius heurippa appears a perfect blend of two others, H. cydno and H. melpomene. Indeed, when the researchers attempted to recreate H. heurippa in the lab by breeding the other two species together, they produced offspring that, after three generations, consistently showed the H. heurippa color pattern. DNA sequencing showed that the newly created hybrid butterflies were genetically distinct from H. cydno and H. melpomene.

The researchers also discovered how similar hybrids may have become isolated in nature. When H. heurippa males had a choice between mating with a H. heurippa female or with a H. cydno or H. melpomene female, the males were 75% to 90% more likely to choose their own kind, the researchers report in the 15 June issue of Nature. Other tests indicated that the apparent hybrids recognize other members of its own species by color pattern alone. "For Heliconius, color pattern is everything," says Marávez.

Evolutionary geneticist James Mallet of University College London in the United Kingdom says the evidence that H. heurippa is a product of hybridization is "pretty convincing." But others remain skeptical. "Maybe this is a hybrid species, but I'm not convinced from the genetic data that it is," says evolutionary geneticist Jerry Coyne of the University of Chicago in Illinois, who says much more sequencing would need to be done to prove H. heurippa is a hybrid. Coyne believes hybridization can't be common in nature because current animal family trees would reveal such mixes.

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