A narwhal is an Arctic whale with a very long tusk (a tooth) on its forehead. From ScienceDaily:
Low genetic diversity has historically been viewed as a species’ death sentence because it was thought that when members of a species have less DNA variation for natural selection to act on, they would struggle to adapt to changes in their surroundings. But this research suggests it might be more complicated than that.
“There’s this notion that in order to survive and be resilient to changes, you need to have high genetic diversity, but then you have this species that for the past million years has had low genetic diversity and it’s still around — and is actually relatively abundant,” says Eline Lorenzen, an associate professor and curator at the Natural History Museum of Denmark. Currently, the population estimate of narwhals places them at around 170,000 individuals, enough to change their IUCN Red List status from “Near Threatened” to “Least Concern” last year.
“This shows us that just looking at the number of individuals isn’t indicative of the genomic diversity levels of a species, but also looking at the genomic diversity levels isn’t indicative of the number of individuals. Equating those two doesn’t seem to be quite as simple as previously thought,” Lorenzen says.
Interestingly, the low genetic diversity found in narwhals appears to be unique to the species; several other Arctic species, including their closest relative, the beluga, all have higher levels of genomic diversity. Paper. (access?) – (open access) More.
Here’s a possible history:
So what, exactly, is going on with the so-called “unicorns of the sea”? A population boom tens of thousands of years ago might be key to understanding the species puzzling lack of diversity, the study authors suggest. Through scientific modelling, the team was able to determine that narwhals began to experience a slow but constant population decline around two million years ago; by 600,000 years ago, only around 5,000 individuals were left. Narwhal numbers began to pick up around 100,000 years ago, roughly coinciding with the onset of the last glacial period, which in turn suggests that the population uptick was caused by “an environmental driver, possibly linked to an increase in Arctic sea ice,” the researchers write. Then, between 30,000 and 40,000 years ago, narwhal numbers started rising rapidly—and, according to the researchers, “genetic diversity may not have had time to increase accordingly.”
Narwhals may have been able to fare well despite their loss of genetic diversity because the population decline that began millions of years ago happened slowly, giving the animals time to “evolve different mechanisms to cope with their limited genome,” says Michael Vincent Westbury, lead study author and a postdoctoral researcher at the Natural History Museum of Denmark. Brigit Katz, “Narwhals Have Low Genetic Diversity—and They’re Doing Fine ” at Smithsonian Magazine
It turns out that low genetic diversity is not necessarily a death sentence. It depends on what else is going on, perhaps.
By the way, we still don’t know why the narwhal has a three-metre tusk (tooth).
See also: Darwin vs. the polar bear
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