Low genetic variation
Genetic variation is the raw material of evolution. Without genetic variation, a population cannot evolve in response to changing environmental variables and, as a result, may face an increased risk of extinction. For example, if a population is exposed to a new disease, selection will act on genes for resistance to the disease if they exist in the population. But if they do not exist if the right genetic variation is not present the population will not evolve and could be wiped out by the disease.
As an endangered species dwindles, it loses genetic variation and even if the species rebounds, its level of genetic variation will not. Genetic variation will only slowly be restored through the accumulation of mutations over many generations. For this reason, an endangered species with low genetic variation may risk extinction long after its population size has recovered.
The risk of extinction or population decline because of low genetic variation is predicted by evolutionary theory. Scientists have not yet found any absolutely clear-cut examples of this in endangered species today, but they continue to investigate the possibility. A case study of the cheetah, which has famously low genetic variation, suggests the sorts of dangers that are possible. When the captive felines at an Oregon breeding colony for large cats were exposed to a potentially deadly virus, it swept through the cheetah population, killing about 50% as a direct or indirect result of the virus but none of the lions even developed symptoms.
Although this example is by no means conclusive, it is possible that the cheetahs' low genetic variation unlike the lions' more extensive variation meant that none of them had the right immune system gene variants to fend off the disease. Similar epidemics could sweep through other vulnerable species with low genetic variation, increasing their chance of extinction.
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Cheetah and lion photos by Gerald and Buff Corsi © California Academy of Sciences
Understanding Evolution © 2017 by The University of California Museum of Paleontology, Berkeley, and the Regents of the University of California