Isle Royale is 200 square miles of land in the watery expanse of Lake Superior. One cold winter 70 years ago, wolves came over an ice bridge and settled into a largely isolated island existence. Unfortunately, island life has not been good for them.
By 2016, the number of wolves on Isle Royale declined from a peak of 50 to just two, a male and a female. As a result of inbreeding, they were half-siblings as well as father and daughter. They had a pup together that lived less than a year. Even before that, scientists were finding wolves on Isle Royale with crooked spines and extra ribs.
The wolves of Isle Royale inspired Chris Kyriazis and his colleagues at UCLA to simulate animal populations over hundreds of generations. Their findings were counterintuitive: What doomed the wolves is not just the small number that have lived on the island in modern times, but perhaps also the large number of wolves that lived thousands of years ago. Kyriazis presented his study at the Evolution 2019 conference, and the team posted a preprint of the article, which has not been peer reviewed yet, on bioRxiv.
A large ancestral population can lead more quickly to extinction, the authors argue, because harmful but recessive mutations are not purged over thousands of years. The chances of any one individual getting two copies of the mutation is low, so natural selection doesn’t get a chance to act on it. But if the breeding population then dramatically shrinks—as when the wolves of Isle Royale isolated themselves from wolves on the mainland—those harmful mutations start to come into play.