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A study of songbirds on Réunion, a small French island in the Indian Ocean, has delved into how and why one species split into four distinct forms in such a tiny space.
The Reunion grey white-eye—a small bird that looks a little like a wren—lives in four distinct population groups on the island. One, with two color variations, lives high up on the island’s mountains. Three other forms live in the lowlands, separated only by two rivers and an ancient lava flow that they could easily fly over.
The presence of four unique forms on such a small island has perplexed researchers for years. Scientists since Charles Darwin have noted how species can evolve into separate, but highly related, species across small areas with differing environments, like the Galapagos Islands. However, evidence for divergence into separate species all within the same region, like one small island, is scant. Often, closely related species may exist together in the same area because they evolved elsewhere, separately, before making contact again. In rare cases where they coevolved together in the same area, scientists usually discover it long after they split into different species.
“Part of what’s cool about this study is they’re presumably catching [the birds] in the act” of evolving into different species, says Ethan Linck, a researcher at the University of New Mexico who studies speciation in birds and was not involved in the study. “Studying speciation is hard,” says Linck, adding that this is arguably the smallest island on which researchers have spotted this type of speciation.
Borja Milá, an evolutionary biologist at the National Museum of Natural Sciences in Madrid, spent over a decade collecting data from all four forms of Reunion grey white-eyes. His team used badminton-like nylon nets stretched between tall poles to safely capture flying birds. They recorded the size and feather color of each bird, and took blood samples to see how genetically related—or distinct—they were.
The team found that the four forms of Reunion grey white-eyes differ not only in color and body size, but also genetically. Their analysis reveals two main waves of dispersal in the birds’ evolutionary history. Sometime after their common ancestor first arrived on the northern tip of Réunion, birds moved south to other low elevation areas and split into different groups. Later, some birds ventured to higher elevations.
The birds are good flyers—their ancestors originally made a 170-kilometer crossing from Mauritius to Réunion—but now they tend to travel only short distances. “The birds just don’t move on the island,” says Maëva Gabrielli, a PhD student at the Evolution and Biological Diversity laboratory in France who led the analysis. They have small territories and only mate with birds that are similar in feather color or song, she says. The highland birds have evolved to be larger, perhaps due to the shrubby habitat and different food at higher elevations.
For now, these four forms are just population groups, possibly in the process of diverging into new species. “But it’s unclear whether this is going to come to fruition,” says Linck.