"Mountain Gorillas and Inbreeding: The Path to Extinction or Salvation?"

A recent analysis of the mountain gorilla genome has revealed something important:

Mountain gorillas are currently suffering from a serious inbreeding problem, and while scientists everywhere worry that this will lead them down the path to extinction, one new genetic study says that it might in fact be their salvation. This is the first study ever to sequence whole genomes from mountain gorillas, and it is helping shed light on the impact of population decline on these critically endangered apes.

Back in 1981, the number of mountain gorillas living in the Virunga volcanic mountain range - on the borders of Rwanda, Uganda and the Democratic Republic of Congo - plummeted to a mere 253 apes, thanks to habitat loss and hunting. Since then, conservationists have been working to restore their population, boosting their numbers to approximately 480 among the Virunga population.

However, while this is certainly an improvement, these mountain gorillas are not out of the woods yet. Scientists are concerned that severe inbreeding among this small population is causing decreased genetic diversity and threatening their very survival. But now, researchers are saying that many harmful genetic variations had in fact been removed from the population through inbreeding, and that mountain gorillas are genetically adapting to surviving in small populations.

"This new understanding of genetic diversity and demographic history among gorilla populations provides us with valuable insight into how apes and humans, their closely related cousins, adapt genetically to living in small populations," said Dr. Aylwyn Scally, corresponding author from the Department of Genetics at the University of Cambridge. "In these data we can observe the process by which genomes are purged of severely deleterious mutations by a small population size."

In the study, published in the journal Science, researchers were able to sequence the whole genomes of seven mountain gorillas for the first time using blood samples collected over several years. Previously, only poor-quality DNA from fecal and hair samples was available. This time around, scientists were able to compare the genomes of all four gorilla subspecies. They found that these mountain gorillas - along with eastern lowland gorillas, their closely related neighbors - were two to three times less genetically diverse than gorillas from larger groups in western regions of central Africa. What's more amazing is that mountain gorillas have managed to survive in small populations for thousands of years.

It has long been a concern that for a species that is already critically endangered, severe inbreeding would push them more towards extinction - that is, with low level of genetic diversity making them more vulnerable to environmental change and disease, including cross-infectious strains of human viruses. However, that does not appear to be the case. The inbreeding has, in some ways, been genetically beneficial. For example, fewer harmful loss-of-function variants were found in the mountain gorilla population than in the more numerous western gorilla populations. These variants stop genes from working and can cause serious, often fatal, health conditions.

"We worried that the dramatic decline in the 1980s would be catastrophic for mountain gorillas in the long term, but our genetic analyses suggest that gorillas have been coping with small population sizes for thousands of years," concluded Dr. Yali Xue, first author from the Sanger Institute. "While comparable levels of inbreeding contributed to the extinction of our relatives the Neanderthals, mountain gorillas may be more resilient. There is no reason why they should not flourish for thousands of years to come."

That aside about Neanderthals is conjecture. Anyway, this is something people have been saying here and there for a while, but this is the first hard evidence showing it to be the case. Because mountain gorillas have had smallish, isolated populations for most of their history due to the terrain, they’ve had to deal with a higher than average level of endogamy than other species. The combination of gene mixing between populations and gene isolation within populations, ensured that harmful recessive genes would be eliminated faster from the gene pool, and that this process wouldn’t destroy the population as a whole. As a result, when humans started driving them to near extinction, their gene pool was already adapted to deal with high levels of mating among close genetic relatives. (This happened to cheetahs as well.) This means that the sparsely populated mountain gorillas are actually more likely to survive human destruction and poaching than their cousins at sea level.

So is endogamy good or bad? It depends. Like most things in life, it depends.