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Island study reveals animals can diversify in very small regions

With the discovery of four new types of mice on Mindoro Island in the Philippines, a team of scientists has identified the smallest known island where several species of animals have evolved from one single species.

Study co-lead author Lawrence Heaney is the Negaunee Curator of Mammals at Chicago’s Field Museum.

“The single most remarkable thing about planet Earth is there are so many species here, so much biodiversity. We take it for granted, but holy cow, there’s a whole lot of stuff out there – how did it get here?” said Heaney.

“This is one of the few papers ever written to look at whether there’s a limit to how small an island can be for species diversification to occur, and it’s the only one looking at it in mammals. Mindoro is by far the smallest island on which we’ve seen this happen.”

When animals are isolated on islands, they diversify into unique new species.

“There are many islands that have species that arrived from somewhere else and that subsequently changed into something distinctive. Many of these islands are much smaller than Mindoro,” explained Heaney.

“Rather, the key to this study is whether a single species that arrived from somewhere else has produced multiple species that all evolved within the given island from the single ancestral species. It is the issue of an increase in the number of species within the island, by evolution within the island.”

Prior to this study, the smallest island where scientists knew mammal species had diversified was Luzon, the largest island of the Philippines.

Heaney said his team wanted to find an even smaller island that supported evolution within a species. “We looked at a map and said, okay, where’s there a smaller island where diversification may have occurred?”

The scientists set their sights on Mindoro, which is a tenth the size of Luzon and around two-thirds the size of Connecticut. Study co-author Danny Balete, who is now deceased, led four seasons of fieldwork missions on Mindoro in search of the island’s mammals.

“The mice we looked at in this study are all members of the ‘earthworm mouse’ group Apomys – they love earthworms, but they also eat seeds and fruits. They’ve got big dark eyes, great big ears, long soft fur, white feet, dark tails–they’re very pretty little mice,” said Heaney.

DNA analysis revealed that the earthworm mice belonged to four separate species, three of which were previously undiscovered. The research also showed that all four of the species had branched out from a common ancestor on Mindoro Island.

“The results are unambiguous – we’ve got four species of forest mice on Mindoro from one colonization event from Luzon about 2.8 million years ago, and three of those four mouse species are found on their own separate mountains.”

Mindoro Island is the same size as Yellowstone National Park, which means that new mammal species can evolve from one ancestor in areas as small as some wildlife preserves.

“This study changes how I think about conservation,” said Heaney. “When we think about how to design protected areas, we need to think about the topography of the Earth, not just a flat map. The fact that these mice evolved on their own separate mountains within a limited geographic area tells us that mountains are important.”

The study is published in the Journal of Biogeography.

By Chrissy Sexton, Staff Writer

Image Credit: L. R. Heaney, The Field Museum

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