Harvard University evolutionary biologist Naomi Pierce began studying Lycaenidae butterflies as a graduate student in the 1980s. But only recently has she accumulated enough data, from her team’s work and others’, to begin to address the question that has long dogged her: Why is this group of butterflies so diverse?
In a new study published in Proceedings of the Royal Society B, her team provides some clues, showing how being specialized can influence the trajectory of evolution. “In a very simple way, if you are more specialized that can promote more biodiversity,” says Pierce of the new findings. “And that is why there are so many of these butterflies.” The family comprises about 5,000 species, about 30% of butterfly species on the planet.
Lycaenidae caterpillars are known for their specialized associations with certain plants or ants, with some species tending to the insects with delectable secretions from their bodies. In turn, the ants can protect against spiders, wasps, and birds, biting them or spitting acid on them. The ants “are pretty savage,” says Pierce.
In the new study, Pierce and her colleagues examined 967 species of Lycaenidae found worldwide, looking at factors such whether the species relies on ants from a single genus for survival (“ant-specialized”) or eats only plants from a single genus (“plant-specialized”), or are generalists (not reliant on specific ants or plants). They correlated these lifestyle data with genetic data obtained from sequencing databases of a mitochondrial gene. They then supplemented these data with their own mitochondrial DNA sequencing efforts, examining at least five individuals of each species.
The researchers went on to compare specialist species with generalist species. They found that, in general, specialized species had a greater variety of genetic changes in the DNA and had greater genetic differences between populations. This finding held for ant-specialized and plant-specialized species, as well as a third category, species that have evolved to eat ants, an even more specialized group. Says Pierce, “These are population genetic measures that might be associated with more rapid rates of diversification.”
The researchers note that evolutionary selection may be driving some of this pattern or it might result in part from isolation of smaller populations of specialists, marooned on ecological islands with specific plants or ants—a process otherwise known as genetic drift. Further studies—for instance detailing the level of isolation of various populations, along with more extensive genetic analyses—might help shed light on this open question, says Douglas Futuyma an evolutionary biologist at Stony Brook University in New York.
Even so, the study goes on to provide insight into an ongoing debate about the forces behind what evolutionary biologists call macroevolution, which involves processes such as the formation of new species, says Zach Gompert, an evolutionary biologist at Utah State University in Logan. Study researchers showed that specialization correlated roughly with higher rates of molecular evolution (which involves DNA base changes) over long time scales, a macroevolutionary pattern. This effect was most pronounced for the most specialized group, species that eat ants.
There is a “pretty big fight” about whether the factors that promote diversity, variation, and genetic differentiation within species are the same things that give rise to macroevolution, says Gompert. “This study is showing that to some extent the answer is yes,” he says.
The findings not only help explain the high diversity of Lycaenidae, but also could provide insight into diversity of other groups of organisms that have specialized members. “The things that make it a powerful study,” adds Gompert, “are the scope and scale of the sampling.”