All polar bears sitting in the arctic, eating large seals.
Polar bears end up drifting off on an iceberg, barely surviving as they move along.
Polar bears end up landing on an island, not being able to reach the seals and barely surviving, relying on smaller animals.
One of the polar bears mutates, making it smaller and having wings, which allows it to sustain a diet off of fish. B/C of this, the polar bear has better fitness.
Eventually, this mutation passes on to almost all of the rest of the polar bears, since the ones with wings can more easily reproduce than others.
A few of the polar bears return back to the cold environment, as they blend in better there. The two different species now do not breed anymore.