In Ontario, in the winter of 1997, a particularly virile male wolf stepped onto the ice of Lake Superior and headed toward Isle Royale, an island about 15 miles offshore. There he radically changed the genetic makeup of an isolated group of wolves that had lived there since the late 1940s.

Researchers, who for many years have been observing the Isle Royale packs and the moose they feed on, did not realize at first that he was an immigrant, but soon his appearance and behavior became impossible to ignore.

He was larger than most of the Isle Royale wolves, and was so strongly territorial that he completely displaced one of the four packs, driving it to extinction within two years of his arrival. His own pack grew to 10 wolves, the largest seen on the island in almost 20 years. As he aged, his fur grew paler, almost white, a phenomenon known in other wolves but never before seen in the Isle Royale animals.

“We don’t know of any other instance — except when they first came — of wolves crossing the ice,” said John A. Vucetich, the lead author of a study of the wolves published online in The Proceedings of the Royal Society B last week. “The entire population is descended from a single female.”