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.
Early genetic analyses didn't support that idea. Studies that looked at pieces of mitochondrial DNA, a type of genetic material that can be used as a molecular clock to measure the time since two genetic lineages split, concluded that the various killer whale groups are similar enough to fall into a single species.
But recently, researchers have come to realize that not all molecular clocks keep the same time. The mitochondrial DNA of Adelie penguins, for example, evolves faster than previously thought (SN Online: 11/17/09). Killer whales and other cetaceans, on the other hand, have molecular clocks that tick more slowly than other species' clocks do, says Phillip Morin, a marine mammal geneticist at the Southwest Fisheries Science Center in La Jolla, Calif.
Morin and colleagues analyzed the mitochondrial genomes of 139 killer whales from around the globe and found that the animals fall into several genetically distinct groups.
"The genetic data show that they are each independently evolving lineages," Morin …