Most orca populations declined during the Ice Age, and this bottleneck is recorded in their DNA. But at least one population, off the coast of South Africa, was apparently unaffected.
Organisms hold a record of their species’ history in their DNA. By analyzing small differences in the genetic sequences of individuals within a species, scientists can determine how populations might have mixed or become isolated, grown or declined over time.
Andre Moura of Durham University in England and colleagues examined the historical population dynamics of killer whales through analyses of the whole genomes of a couple of orcas from the Northern Hemisphere combined with analyses of the mitochondrial DNA of whales from around the world. The orca populations they looked at were in South Africa, Antarctica and the North Atlantic as well as resident and transient cetaceans in the North Pacific. This represented most of the world’s populations of killer whales.
The orca DNA showed that the animals survived most of the Pleistocene (2.6 million to 11,700 years ago) just fine, even though there were more than a dozen cycles of glacial advance and retreat during that time. The last ice age, though, was a doozy. Something happened during that period that led to a huge decline in genetic diversity in most orca populations. That probably means a huge drop in population numbers. Only the South African population seems to have survived that period unscathed; that population is about 10 times more genetically diverse than the others, the researchers report February 4 in Molecular Biology and Evolution.
The most recent glacial period was special in a couple of ways: It had the lowest recorded temperature, and climate was more unstable than in previous ice ages. There were also changes in the upwelling off South America, Northern Africa and the West Coast of North America during that time, which may have led to declines in marine productivity and resulted in less food available for orcas.
Upwelling brings cool, nutrient-rich water from deep in the ocean to the surface, where it promotes the growth of seaweed and phytoplankton. That increased growth filters up the food web and supports the growth of populations of fish, birds, mammals and other organisms.
But during the Ice Age, upwelling didn’t change in the region where the South African orcas live, and marine productivity didn’t decline. That would have provided stability for this population of killer whales. And it shows how important small zones of stability can be for the survival of species during times of changing climate.