The other side of inbreeding
Inbreeding is generally regarded as bad for species, if not downright icky. But something called “purging” has sometimes been invoked to explain why some populations squeeze through a bottleneck and blossom on the other end with few problems. Now some fly experiments have confirmed the power of purging – at least in the lab.
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Inbreeding is generally regarded as bad for species, if not downright icky. But something called “purging” has sometimes been invoked to explain why some populations squeeze through a bottleneck and blossom on the other end with few problems. Now some fly experiments have confirmed the power of purging – at least in the lab.
When only a small number of animals remain in a population, there is always a worry that inbreeding will bring out unpleasant recessive genes. The reduction in the reproductive vigor of an inbred population is called “inbreeding depression” by conservationists. But not all species seem to show it to the same extent.
“We can have populations that lack genetic diversity and do not seem to show a decrease of fitness,” says Juan Bouzat, who presented the experiments today at the Society for Conservation Biology meeting in San José, California. An example of this is the Northern Elephant Seal, which passed through a very tight bottleneck due to hunting, and now seems fine. The common explanation, according to Bouzat, is that this is because of a “purging” of the bad genes by natural selection.
Here is how purging is thought to work: in inbreeding, the chances of two copies of a deleterious gene showing up increase, and those recessive genes that have been quietly hanging out in the population emerge to be acted on by natural selection. If they are deleterious enough, they will doom the animal and become “purged” from the population. If this happens to a population, then the next time they are put though a bottleneck, there will be fewer bad recessives to show up and decrease the animals’ fitness. The prediction is, therefore, that a history of inbreeding will make a species better able to pass through another genetic bottleneck without going extinct.
Bouzat, a biologist at Bowling Green State University in Ohio and William Swindell, his student, put this idea to the test by breeding two populations of fruit flies caught in an orchard in Bowling Green. One population they put through a bottleneck, they other they did not. Then, in each population, they measured how much worse off the offspring of siblings were than the offspring of unrelated individuals.
Their key result was that populations that had never been inbred had a much worse time when forced to breed with their siblings—they produced far fewer viable offspring. In the group that had been inbred , a sibling match was still less successful than a random one, but not as dramatically so. It was as if, having shed some unpleasant genetic baggage during their former period of inbreeding, there was less to go wrong.
So “inbreeding depression” may not effect all species equally. “In reality,” says Bouzat, “different species may have different probabilities of going extinct , depending on their inbreeding history.”
University of Vermont population geneticist Charles Goodnight says the result is important for conservationists. “If you have a species forced to a very small size , if you keep the population safe and viable through the risky inbreeding depression period, it could improve in the end.”
References
1. Swindell, W & Bouzat, J. Journal of Evolutionary Biology. 19 (1257-1264). 2006
2. Swindell, W & Bouzat, J. Evolution. 60 (1014-1022) 2006.