Determining whether animals belong to the same species is not as black and white as you might think.
Take killer whales. Scientists have long debated whether the ocean-dwelling mammals all belong in one species. Now, DNA evidence suggests that killer whales should be classified in at least four species, and maybe more.
Scientists once thought killer whales all belonged to the species Orcinus orca. But as researchers began observing more closely, they discovered that the whales seem to belong to different groups, called ecotypes, with distinct feeding habits and appearances. Killer whales from different ecotypes don’t seem to breed with each other — one criterion for being classified as separate species. So some scientists proposed that killer whales should be grouped into different species.
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