How Evolution Works: Selection of Rare Individuals and Lots of Death

This makes perfect sense to me:

In their research, scientists from Canada and Europe removed marine stickleback fish from the ocean, put them into ponds with gradually dropping temperatures, and studied them for three years.

Over three generations, one per year, the fish evolved to survive water 2.5 degrees Celsius below the limit for their great grandparents... (snip)

"But just because we've seen a large evolutionary response, that doesn't mean a natural population can adapt to climate change with no consequences," Barrett told AFP Thursday.

About 95 per cent of the fish population died during the three-year study, with only five per cent developing a tolerance for cold," he said.