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.
As Science evolved, so did the killer whales…
However, a new whole-genome study of the predator’s DNA has recently confirmed that killer whales do indeed come in more varieties than anyone had realized. The finding could have important implications for marine conservation efforts, such as reducing the number of killer whales allowed to be caught when fishing in certain areas.
Until recently, scientists thought orcas that freely roamed the cold waters of the northern Pacific Ocean and the Bering Sea were a single species. But studies of the killer whales’ behavior and genetics suggested that there are at least two species — “resident” fish-eating orcas and “transient” mammal-eaters also known as Bigg’s killer whales (SN: 5/22/10, p. 8).
A new study of 462 killer whales, published July 11 in the Journal of Heredity, supports the division of northern Pacific orcas into two species. DNA analysis also indicates those species aren’t monolithic, Kim Parsons of the National Oceanic and Atmospheric Administration and colleagues report. Resident orcas fall into four genetically distinct subpopulations; at least five subgroups of transient killer whales ply the frigid seas.
Those subdivisions largely reflect the prey each group prefers, the researchers say. This whole scenario illustrates the plasticity and intricacies of evolution and how subtle genetic differences between almost identical organisms can actually translate to them being entirely different species. These conversations inevitably lead to questions regarding humans and their evolution and in particular, to whether humans are still evolving in this day and age!
Are humans still evolving?: How to answer like an undergraduate
These questions require a clear and strong understanding of what evolution actually means and an interviewee should be ready to deploy a concise but accurate definition of the term in an interview situation. In this case, the idea that evolution is the change in gene frequency over time should be stressed. Many students will claim that humans have become so advanced that we are no longer subject to any selection pressures as we are so able to mould our environment to suit our needs instead of needing to adapt to our surroundings. However, they would be foolhardy to forget that humans still face a myriad of selection pressures, especially with regards to health and disease. For example, the gene frequency of alleles that confer resistance to the development of AIDS following HIV infection has been increasing amoung populations where the disease is common showing that even over a relatively quick timescale, humans are indeed still evolving. Before next months blog, see if you can dig up any further examples of recent human evolution.