Killer whales are one of the most widely distributed mammals on Earth. These cosmopolitan cetaceans arose millions of years ago and have persisted through myriad threats—from the dramatic climate shifts of the last ice age, to the hunters who aggressively pursued them until the 1980s. But behind their resilience lurks a danger that threatens their hard-fought existence: many killer whale populations have low genetic diversity.
“The reason for this actually goes way, way back into their past,” says Andrew Foote, an evolutionary biologist at the Norwegian University of Science and Technology and the leader of a new project that pieced together the genetic history of killer whales from across the globe.
During the tail end of the last global glaciation, pods of killer whales spread out from ice-free areas near the equator, setting up shop in once uninhabitable waters. Ever since, some of these populations have evolved largely in isolation. In a pair of recent papers, Foote and his colleagues show how these cetacean trailblazers established what eventually became islands of stagnating genetic diversity. “Essentially,” says Foote, within some of these populations, modern killer whales “all have the same DNA as those colonizers,” even after thousands of years.
To come to this result, Foote and his team worked with researchers around the world to assemble a data set of 26 genomes from different killer whale populations—from North Atlantic pods to those living off Antarctica. They used genomic sequencing to map out entire sets of genes from individual killer whales, then extrapolated the results to the population level. In particular, the team looked for long strings of genes that had identical copies from the mother and father, an indication of inbreeding within a population. The killer whales that live near the Earth’s poles, it turns out, are particularly inbred.
Today, some killer whale populations are faring better than others. The new research suggests that—when factoring in differing pressures from hunting, habitat loss, pollution, and other forces—some of that modern success hinges on the makeup of their initial populations thousands of years ago. A slightly larger group of colonizers meant a larger gene pool to work from. It also helped if the original founders’ genes were as diverse as possible. More importantly, those early colonizers had to have begun reproducing prolifically to maintain a large population size. When it comes to maintaining genetic diversity, every little bit of reproductive success in each generation counts.
For isolated groups that are dwindling in size and with few apparent opportunities for the infusion of fresh genetic material over many generations, successive inbreeding spells trouble for future progeny, says Chloe Robinson, an ecologist at the Canadian NGO Ocean Wise, who didn’t participate in the examination. Harmful genetic mutations are building up in the modern descendants of these populations faster than natural selection can purge them from the bloodline. “It’s a bit of a ticking time bomb,” says Robinson.
Axel Janke, a geneticist at Goethe University in Germany who also wasn’t involved in the recent studies, explains that low genetic diversity can weaken a population’s ability to adapt to environmental shocks. Having a homogeneous gene pool is like having fewer plays on hand to survive any curveballs the environment throws at them. If all members have nearly identical genetics, “a single infection can kill that population,” he says.
Looking at two modern killer whale populations shows how these differences can play out over time.
In Norway, for instance, an ancestral pod successfully monopolized the bountiful herring and multiplied quickly, maintaining high numbers over generations. This allowed their population to weather their genetic bottleneck by diluting out dangerous mutations. “Having a big population size has been key to their survival,” says Foote. “Herring was their savior.”
In comparison, the killer whales that settled in the Scottish seas are spiraling toward extinction. The early colonizers didn’t breed as quickly as their Norwegian peers. “They’ve been inbreeding probably for a long, long time,” says Foote. Stuck with a tiny start-up gene pool, the population is floundering—they have produced no new calves in 20 years.
As killer whale numbers off Scotland and elsewhere dwindle further because of pollution, habitat loss, and the decimation of their food sources, the situation is only going to get worse, says Magnus Wolf, an evolutionary geneticist at the Senckenberg Research Institute in Germany who was not involved in the new research. The fallout from a shrinking population is a reduced gene pool, which may accelerate the species’ decline.
“Everybody talks about species loss, but nobody talks about genetic loss,” says Wolfe. We may not see the result of the damage that has already been done until it is too late.
Killer whales, as a species, have been hanging on—some populations, barely—since Earth thawed out from its last glaciation. But in their survival is a warning. With the world warming because of anthropogenic climate change, many species are already beginning their own poleward migrations. History cautions that even if a few particularly itinerant animals can outrun warming waters, it may not be without consequences—even thousands of years down the line.
hakaimagazine.com, 4 November 2021