How Do Species Evolve?

Male beaked whales have fancy tusks. The reason isn't clear. Perhaps they look sexy to females or help males fight off competition. Either way, the differences in tusks helped beaked whales evolve into 21 separate species, according to biologist Merel Dalebout of the University of New South Wales, Australia. She is working with Oregon State University whale expert and National Geographic Society grantee Scott Baker. If generations of females prefer males with one kind of tusk, eventually their descendants can grow so distinct they become a new species, like the dense-beaked whale whose tusks are shown here in an undated photo. (Read the full story.)
—Photograph by Nan Hauser