Determining whether animals belong to the same species is not as black and white as you may think.
Take killer whales, for example. Scientists have been debating for years whether the ocean-dwelling mammals belong to a single species or several different ones. Now, new DNA evidence seems to indicate that killer whales should be classified in at least four different species.
Scientists used to think that Shamu and Willy of the movie “Free Willy” and other killer whales (also called orcas) were all members of a single species, Orcinus orca, which had colonized oceans all around the world. But as researchers began observing killer whales more closely, they discovered that the marine mammals seem to belong to several different groups, called ecotypes, with different feeding habits and appearances. In Antarctic waters, for example, three different ecotypes have been identified that have distinguishing eye patches; dietary preferences for either fish, seals or cetaceans and social networks that don’t intersect.
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 divided into different species.
But early genetic analyses didn’t support the move. Biologists often look at a loop of DNA housed in organelles called mitochondria, which produce energy for cells. Partly because it resides outside the main storehouse of genetic material in the cell’s nucleus, mitochondrial DNA can be used as a type of molecular clock to measure the time elapsed since two genetic lineages shared a common ancestor. After looking at changes in portions of the mitochondrial DNA from different killer whale ecotypes, earlier studies concluded the groups were similar enough to fall into a single species.
But recently, researchers have come to realize that some species may accumulate mitochondrial DNA changes faster or slower than others. Adélie penguins are evolving faster than previously thought (SN Online; 11/17/2009). But killer whales and other cetaceans have molecular clocks that tick slower than in other species, says Phillip Morin, a marine mammal geneticist at the Southwest Fisheries Science Center in La Jolla, Calif. As a result, examining just part of the mitochondrial DNA doesn’t give a full picture of genetic diversity of killer whales.
So Morin and his colleagues analyzed the full 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 says.
-Type B killer whales in the Antarctic feed only on marine mammals, especially seals that they knock off of pack ice. These killer whales have large eye patches and a two-tone grey color scheme. They are medium-size as killer whales go, falling in between the open-ocean dwelling Type A killer whales and the smaller Type C orcas.
-Type C killer whales — also known as Ross Sea killer whales — eat fish and hunt under Antarctica’s pack ice. These are the smallest of the killer whales and have a small, slanted eye patch.
-Transient killer whales that live in the northeastern Pacific near Alaska are the most genetically distinct of the killer whales. These killer whales eat other marine mammals, including seals, sea lions, whales and dolphins. Morin’s group estimates that this group diverged from other killer whales as a separate species about 700,000 years ago. Other groups split off more recently.
Mitochondrial DNA analysis also suggests that Type A killer whales, which feed on other cetaceans, might be further divided into other species once scientists have compiled more data about the animals’ behavior.
“I suspect there’s going to be another four or five species,” says Robin Baird, a cetacean biologist at the Cascadia Research Collective in Olympia, Wash. What is unusual about the killer whales is that they seem to be evolving into new species based upon their eating habits, he says. Other instances of this sort of cultural divide between species are known for birds, but is extremely rare for mammals, he says.
Usually new species emerge after geographical isolation separates two groups into distinct breeding populations. But killer whales don’t have any apparent physical barriers that would keep them from breeding with each other. And scientists have learned that geographic isolation wasn’t enough to divide tree lizards, called anoles, on the Lesser Antilles island of Martinque into new species. Roger Thorpe and his colleagues Yann Surget-Groba and Helena Johansson reported their discovery April 29 in PLoS Genetics.
Not everyone is convinced that the new genetic data show that killer whales are several different species. Mitochondrial DNA is passed from mothers to their offspring and only reflects part of a species genetic heritage, points out A. Rus Hoelzel of Durham University in England. Analyzing the full genetic makeup of killer whales might show more flow of genetic information between killer whale groups than is apparent from the mitochondrial DNA. Exchange of genetic information between the groups would indicate that they aren’t separate species.
Morin also wants to examine the full genetic picture of killer whales, but he thinks the additional data will validate the finding from mitochondrial DNA that orcas belong to different species.
“My gut feeling is that these really are separate species, but the careful scientist in me wants to get a little bit more data from the nuclear DNA to really nail it down,” he says.