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, 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?” asked Dr. Ryder.
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 a common 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 between the animals. But the scientists couldn’t
be sure just how similar the foxes were until technological advancements
made it possible to sequence their entire 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 any 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 while 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 even 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 had 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. They have also faced new diseases introduced by
raccoons and other invasive animals.
Island fox pups on Catalina Island Video by Karl Huggins
Some
researchers are concerned that their genetic similarity could increase
the risk. “If we care about the persistence of island foxes and other
small populations, we should be concerned about low genetic variation,”
said W. Chris Funk, a biologist at Colorado State University.
Writing in the journal Molecular Ecology last month, Dr. Funk and his colleagues suggested one way to help the foxes: move animals between the islands. If the subspecies mix genes by interbreeding, they may be able to increase their variation.
Scientists have already used this procedure, known as genetic rescue, to help other species. In Florida, for example, the resident panther population
has had a difficult time producing new cubs. Researchers introduced
panthers from Texas, and their new genes have helped the Florida panther
population grow.
Based
on his new research, Dr. Wayne said he didn’t think island foxes need
genetic rescue, at least until scientists find strong evidence that they
are suffering because of low genetic variation.
“I
would really hesitate to move foxes around,” Dr. Wayne said. “Each
island is genetically distinct. I would be loath to destroy that.”
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