Foxes That Endure Despite a Lack of Genetic Diversity

The Channel Islands, off the coast of Southern California, are a natural laboratory for a particularly adorable experiment in evolution.

A unique species called the island fox has lived there for several thousand years, their bodies shrinking over the generations until now each is smaller than a house cat. Adult island foxes weigh as little as 2.35 pounds.

Now a team of scientists has discovered another way in which island foxes are extraordinary: Genetically, they are nearly identical to one another. In fact, a fox community on one island has set a record for the least genetic variation in a sexually reproducing species.

Oliver A. Ryder, the director of genetics at the San Diego Zoo Institute for Conservation Research, said the new research posed a biological puzzle.

It’s an axiom of evolutionary biology that low levels of genetic variation put species at risk of extinction. Yet the delicate island foxes are still racing across meadows and bounding up trees.

“How can the island foxes get away with it?” Dr. Ryder said.

The new study, published on Thursday in Current Biology, was led by Robert K. Wayne, a geneticist at the University of California, Los Angeles. Dr. Wayne has been studying the DNA of island foxes since the early 1990s, hoping to understand their remarkable makeup.

“They’re like dodos,” Dr. Wayne said in an interview. “They have no notion of human fear. You can just put them in your lap.”

Some scientists suspect that island foxes are fearless because of a long relationship with humans.

Native Americans first settled the islands about 13,000 years ago, and they may have brought along gray foxes from the mainland. Previous studies indicate that island foxes share an ancestor with gray foxes that lived 9,200 years ago.

It’s unlikely the foxes made the trip on their own; the islands are separated from the mainland by 12 to 70 miles of open ocean. Another clue pointing to human help: Native Americans painted foxes on rocks and gave them ceremonial burials. Foxes may have had a spiritual importance to them.

However the animals arrived on the Channel Islands, they adapted quickly. The oldest island fox fossils date back 7,000 years and show that they were small even then. The Great Shrinking required less than 2,200 years, it seems.

Dr. Wayne has focused on genetic variation among the island foxes. He and his colleagues started off by examining a few genetic markers, finding striking similarities among the animals. But the scientists couldn’t be sure just how similar the foxes were until technological advances made it possible to sequence their genomes.

There are six subspecies, each living on a separate Channel Island. In their new study, Dr. Wayne sequenced the genome of one fox from each of five subspecies. On the sixth island, San Nicolas, they sequenced two genomes.

Like other animals, island foxes carry two copies of each gene, inheriting one copy from each parent. In large populations with a lot of genetic variation, there can be many versions of a given gene. An animal may inherit two varying copies of a gene from its parents.

But the scientists discovered virtually no differences in the DNA the foxes had inherited. “We call it genetic flatlining,” Dr. Wayne said.

While each subspecies has very little diversity, the foxes on San Nicolas are almost like identical twins, a record.

Low genetic variation can pose a serious threat to survival. When a new threat appears — a new disease, for instance — some individuals may have the genes to resist it and others lack them. In a population with low genetic variation, none of the animals may have the right genes to survive.

Inbred populations also often share mutations that are bad for health, shortening life spans or reducing the number of offspring. Dr. Wayne and his colleagues found that island foxes have many more harmful mutations than gray foxes on the mainland.

On the face of it, the island foxes should have vanished long ago. “But that hasn’t happened to them in thousands of years,” Dr. Wayne said. “They’re an exception to the paradigm.”

Dr. Wayne speculated that island foxes might enjoy some sort of special protection. It’s possible, for example, that as top predators on a small island, they didn’t have to face the challenges that other inbred animals do. Or perhaps the people who lived on the islands helped them survive.

Or, while the genes of island foxes may be almost entirely identical, maybe they are activated in the animals in varying patterns. Experiences early in life can program genes to switch on and off, a phenomenon called epigenetics.

“It might also be some combination of all of the above,” Dr. Wayne said.

Other researchers suggest that island foxes have been protected in more familiar ways. Newly arrived foxes could have rejuvenated the Channel Island gene pool with some fresh variation.

“Their study has not ruled out occasional immigrants reaching the islands,” said Richard Frankham, a geneticist at Macquarie University in Australia.

But Dr. Wayne said his new study cast doubt on that possibility.

Each subspecies of island fox is genetically distinct, showing no sign of newly introduced genes moving from island to island. “There’s no evidence of gene flow,” Dr. Wayne said.

Understanding the evolutionary history of the island foxes is important to ensuring they have a future. A rapid population decline led to four subspecies of island foxes being declared endangered in 2004.

It’s likely that several causes helped drive down their numbers.

In recent decades, golden eagles have arrived on the islands and begun killing off the foxes, which have also faced new diseases introduced by raccoons and other invasive animals.