How an island of mice is changing what we know about evolution

Evolution takes time and space. Enough time needs to pass for genetic differences to crop up in a population of animals and make them distinct from their forebears. And enough space is needed to stop interbreeding between populations from ironing out these differences.

But just how much space is needed for species to diversify? Scientists have been puzzling over this question for decades, and it’s one that has taken on increasing importance as human-driven habitat loss constricts the ranges of the world’s wildlife.

In their quest to solve this mystery, a team of researchers from institutions in the U.S. have been scouting the globe for the smallest island that supports multiple lineages of mammals evolved from a common ancestor. And in a new study published recently in the Journal of Biogeography, they describe such a place – an island in the Philippines with a unique array of worm-eating mice.

From one species to four

At around 10,000 square kilometers (4,000 square miles) Mindoro is about the size of Yellowstone National Park in the U.S. and represents a big step down from the previous record-holder, Luzon. Also in the Philippines, Luzon is the country’s largest island and about 10 times the size of Mindoro, which lies just across the Verde Island Passage strait.

When the researchers surveyed the mammals of Mindoro’s forests, they discovered four distinct species of earthworm mice. Nested within the genus Apomys, earthworm mice are found only in the Philippines and get their name from their favorite food.

“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,” said Lawrence Heaney, Negaunee Curator of Mammals at Chicago’s Field Museum and co-lead author of the study. “They’ve got big dark eyes, great big ears, long soft fur, white feet, dark tails —they’re very pretty little mice.”

One of these four species is also found on Luzon, but the other three proved to be new to science when the team analyzed their DNA. These three species appear to only live on Mindoro and the study’s findings indicate they evolved from a small group of mice that travelled over from Luzon sometime between 1.5 and 2.4 million years ago.

But how do tiny mice become seafaring colonists? Easy, says Heaney. They could’ve hitched a ride on some flotsam during a typhoon.

“Given the strong winds and relatively short distances between islands, it is inevitable that on rare occasions, a tree or log with some mice living in it will be floated out to sea and blown across from Luzon to Mindoro,” Heaney told Mongabay. “It could take as little as one day, but certainly not more than a few days – it is not very far.”

Still, while it’s a quick trip from Luzon to Mindoro, the study found all four species on the island likely descended from one small group, indicating Mindoro was probably colonized by earthworm mice just the one time. Heaney said this lucky founding group was greeted by “unlimited resources,” allowing them to quickly proliferate “from a few to dozens, to hundreds to thousands to the current hundreds of thousands.”

These descendants ranged far and wide over Mindoro with some getting up into the island’s mountains. It’s in these mountains where the researchers found the new species, suggesting that mice there became isolated from lowland populations, which ultimately allowed them to evolve into distinct forms.

The researchers think that the fact this diversification happened on mountains may mean geographic features may play a bigger role in evolution than area alone. And not just in the Philippines. Take, for example, the Eastern Arc Mountains that run down eastern Africa. Topped with isolated forests, each mountaintop is home to unique species found nowhere else in the world.

“This study changes how I think about conservation,” Heaney said. “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 presence of mountains is so influential for evolution that Heaney said it could perhaps even be used as a proxy for speciation.

“So, once we figure out how big an island needs to be to have two large, fairly old mountains, we may know the minimum size of island [for mammal diversification],” Heaney said.

The researchers are now focusing their search for still-smaller islands on places where there are at least two mountains; they’re currently eyeing a number of possibilities in Indonesia.

Conservation implications

According to Heaney and his team, the diversification of mice on an island the size of a national park underlines the importance of protected areas not just for species preservation, but for species emergence as well.

“As human population continues to expand, what’s going to happen to everything else? How will new species be able to evolve?” Heaney said. “This project is a step forward in being able to answer that.”

The study may also have implications for the conservation of critically endangered species with small remaining populations. When an animal’s population size becomes very reduced, scientists worry about a phenomenon called “inbreeding depression,” which happens when close relatives breed so many times that harmful genes normally rare in a healthy population become common. These genes can express themselves in ways that can harm an animal or make it unable to reproduce, and ultimately may reduce the fitness of a population and even bring about its extinction.

Conservation geneticists have long argued over how many individuals are needed to maintain enough genetic variability in a population to keep it from collapsing due to inbreeding depression. In the 1980s, researchers set that number at 50. However, conservation successes with populations that initially numbered fewer than 50 have given scientists hope that species with just a handful of individuals remaining might be able to come back from the precipice of extinction.

The study’s results indicate just a few earthworm mice made it to Mindoro and went on to beget four healthy species comprising hundreds of thousands of individuals. So how did they escape inbreeding depression? Heaney thinks it may have something to do with how quickly they proliferated. With lots of food and little competition, the initial population likely didn’t suffer many diebacks, which helped safeguard what little genetic variation it had. Still, he said, their survival may have been touch-and-go for a while until new mutations arose and strengthened their diversity.​

Heaney said a similar option might be possible for other species – if certain conditions are met.

“The lesson that I take from this is that when a sudden, rapid, massive decline in a population occurs (in this case, due to a colonization event), IF the population is then able to grow to larger numbers very rapidly, and to have enough resources to maintain a large population over enough time to accumulate new genetic variation, they may do perfectly well,” Heaney said. “Those are pretty stringent requirements, but it is essentially the scenario that is the hope of many of the captive breeding programs for endangered species that are currently in operation.

“It is a long shot, but it is sometimes the best one we have.”

 

Banner image of an earthworm mouse on Mindoro’s Mt. Halcon. Photo by L.R. Heaney / The Field Museum

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