Wildlife populations are most threatened when their numbers become reduced to the point that their genetic diversity is lost. Their narrowing gene pool can accelerate into what is called an “extinction vortex.” With ever fewer gene variants (alleles), the ability to adapt and evolve declines. As inbreeding increases, deleterious genes accumulate, and fitness plummets. The creatures typically have fewer offspring, many of them physically or behaviorally impaired, susceptible to disease, increasingly incapable of thriving. Most people assume they are doomed, but that no longer has to be what happens.
“Genetic rescue” restores genetic diversity. Conservation biologists are warming to its use with growing proof of its effectiveness. One study of 156 cases of genetic rescue showed that 93% had remarkable success. The most famous case was a dramatic turnaround for the nearly extinct Florida panther. By the mid-1990s only 26 were left, and they were in bad shape. In desperation, conservationists brought in 8 female Texas cougars (which are closely related to the Florida cats). Five of the females reproduced. The result of the outcrossing was a rapid increase in litter success—424 panther kittens born in the next 12 years. The previous population decline of 5% a year reversed to population growth of 4% a year. Signs of inbreeding went away, and signs of increasing fitness grew. Scientists noticed, among other things, that the genetically enriched panthers were becoming harder to capture.
Often genetic diversity can be restored by means as straightforward as connecting isolated populations with wildlife corridors or larger protected areas, but new technological capabilities are broadening the options for genetic rescue. Advanced reproductive technology offers an alternative to transporting whole genetically-distinct parents—artificial insemination has brought genetic refreshment to cheetahs, pandas, elephants, whooping cranes, and black-footed ferrets. With the cost of genome sequencing and analysis coming down, it is becoming possible to examine each stage of genetic rescue at the gene level instead of having to wait for external traits to show improvement. This has already been done with Rocky Mountain bighorn sheep.
Another strategy being considered is “facilitated adaptation.” Different populations of a species face different local challenges. When a particular population can’t adapt fast enough to keep up with climate change, for example, it may be desirable to import the alleles from a population that has already adapted. With gene editing becoming so efficient (CRISPR etc.), the desired genes could be introduced to the gene pool directly. If necessary, the needed genes could even come from a different species entirely. That is exactly what has been done to save the American chestnut from the fungus blight that killed four billion trees early in the 20th century and reduced the species to functional extinction. Two fungus-resistant genes were added from wheat, and the trees were made blight-proof. They are now gradually in the process of being returned to their keystone role in America’s great eastern forest.
One further reservoir of genetic variability has yet to be employed. In museums throughout the world there are vast collections of specimens of species that have been reduced to genetically-impoverished remnant populations in the wild or in captive breeding programs. Those museum specimens are replete with “extinct alleles” in their preserved (though fragmented) DNA. Ancient-DNA sequencing and analysis is becoming so precise, the needed alleles can be identified, reproduced, and reintroduced to the gene pool of the current population, restoring its original genetic diversity. The long-dead can help rescue the needful living.