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For Endangered Species, a Call for Genetic Rescue

Posted by Richard Conniff on December 12, 2014

Before God told Noah to take “two of all living creatures, male and female” into the ark with him, He probably should have consulted with a wildlife biologist. Then He’d have known that extensive inbreeding after the flood would cause the rapid extinction of many of the species Noah had built his ark to save.

We are in roughly the same boat today. Instead of divine floodwaters, the relentlessly rising tide of human civilization is spreading into every corner of the landscape, leaving populations of threatened or endangered species isolated in a few remaining islands of habitat.  These survivors—tigers in India, red wolves in North Carolina, the kakapo parrot in New Zealand, African wild dogs in South Africa, and countless other species—almost inevitably experience inbreeding and reduced fitness, a recipe for extinction.

But a new paper published in the journal Trends in Ecology & Evolution argues that “genetic rescue” could provide a fix for inbreeding problems.  It’s not about genetic engineering—no Franken-Kakapo—but rather about the seemingly simple business of crossbreeding with individuals introduced from outside populations.

“Genetic rescue has the potential,” conservation biologist David A. Tallmon and his co-authors write, “to be one of the most powerful means to

conserve small and declining populations,” and it has already demonstrated its power, notably in the case of the Florida panther.  “Yet in practice, it remains controversial and is rarely applied.”  The study suggests that cautious conservation biologists tend to exaggerate the risks of genetic rescue.  They may thus be missing out on an essential tool to keep wildlife populations healthy long enough to resolve the underlying problem that caused them to become isolated in the first place.

Understanding how “genetic rescue” works is about as basic as high school biology: In any population, there are different varieties, called alleles, of every gene. Some of these alleles have mutations that can cause health problems. Fortunately, these “deleterious” alleles are usually recessive: As long as an animal has one of each kind of allele for a given gene—that is, one good, non-mutated allele and one mutated, recessive allele—it stays healthy. But in a small, inbred population, the chances of an individual getting two copies of the deleterious allele dramatically increase. As a result, that individual is likelier to die young.  Or it may fail to reproduce, because it’s not strong enough to fight for a mate, or because physical deformities reduce fertility.

That’s what was happening to Florida panthers in the 1980s and early ‘90s.  As their habitat and population shrank, the cats began showing signs of inbreeding depression, including kinked tails, undescended testes, and other health problems. In 1995, scientists decided that the roughly two-dozen remaining Florida panthers needed genetic rescuing. They brought in eight female cougars from a Texas population of a different subspecies, but closely related to the Florida panthers. Within six years, the Florida panther population was showing signs of recovery. Cats with Texan ancestors had normal testes and fewer kinked tails. These days, the population of Florida panthers is somewhere between 100 and 180 and climbing.

So why don’t biologists rely on genetic rescue more often?  One big worry is the risk of outbreeding depression.  In some cases, a small, isolated population of a species may have become especially well adapted to local conditions.  That’s how evolution of a new species begins.  But if individuals arrive from another population with different adaptations, cross breeding might just un-do evolution, resulting in offspring that are less well adapted to local conditions. That could lead to lower survival rates.  A rescue that was intended to slow the demise of the population could end up hastening it instead.

In the new paper, Tallmon and his-co-authors examined 18 documented instances of genetic rescue, in species ranging from wood rats to Mexican wolves. They found that in almost all cases—including experiments with bighorn sheep, snakes, and prairie chickens—population size increased after one or more individuals from another population arrived and bred with the isolated population. Only a single case, an experiment with tiny Pacific Coast marine invertebrates, called copepods, resulted in outbreeding depression. “Genetic rescue appears to be much more important and powerful than we anticipated,” said Tallmon, a professor at the University of Alaska Southeast.  Moreover, modern genetic testing techniques make it possible to reduce the risk of outbreeding depression even further. It’s now relatively easy and inexpensive to identify the genes that are causing problems in an isolated population, then introduce only outsiders with healthy alleles for that gene.

While the risks from outbreeding depression may be overblown, the other problem with genetic rescue is that it is only a temporary solution, said Philip Hedrick, of Arizona State University. “This is a short-term bandage,” he said. In a study published earlier this year, he looked at genetic rescue in an isolated population of wolves on Isle Royale in Lake Superior.   In 1997, a single wolf crossed an ice bridge and joined a pack there. That helped produce a temporary increase in population to about 30 animals by 2006, but after two or three generations, the benefits of crossbreeding faded away.  The population of wolves on the island is now down to just a dozen or so individuals.

But even if introducing outsiders cannot permanently prevent the extinction of a population, it can delay it.  That’s the real value of genetic rescue as a conservation tool: It can stave off extinction long enough to address the root cause of the initial decline. Often, that cause is habitat loss or fragmentation, said Tallmon, and the survival of a healthy population can be the most powerful argument for getting government officials to reverse that process.

In the case of the Florida panthers, for instance, genetic rescue kept the population going long enough for the state government to protect more than 300,000 acres of prime habitat and create highway underpasses to protect the big cats from cars.  Elsewhere conservation biologists are working to re-establish corridors between isolated populations, so the animals can go about cross breeding on their own.

Otherwise, as one scientist wrote in a 2010 commentary on the Florida panther genetic rescue, “wildlife managers all over the world can look forward to carting rare species from one park to another until the end of time.”

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