The hummingbirds that live on the east slope of the Andes Mountains in Ecuador have adapted to a range of habitats, from steamy lowland rainforest to windswept alpine meadows. Biologists Smith and Schneider, studying the differences between these adjacent populations, are finding that natural selection in different ecological niches has pushed the birds down different evolutionary paths and created new species, even though the populations have not been isolated from each other geographically. From Evolution: "Darwin's Dangerous Idea."