When hunters encountered a hybrid of a polar bear and a grizzly in 2006, Kelly's colleagues remarked that the incident was just a fluke. But as Kelly delved into the issue, he found more evidence of similar anomalies. In 2009, a cross between a bowhead and a right whale was spotted in the Bering Sea, between Alaska and Russia. And a museum specimen in Alaska attests to breeding between spotted seals (Phoca largha) and ribbon seals (Histriophoca fasciata), which belong to different genera, a scientific classification of organisms that is broader than the species level.
Evidence suggests at least five other types of hybrids that may arise from animals of distinct genera, Kelly's team reported. These include:
- Narwhal (Monodon monoceros) and beluga whale (Delphinapterus leucas)
- Ringed seal (Phoca hispida) and ribbon seal (Histriophoca fasciata)
- Bowhead whale (Balaena mysticetus) and right whale (Eubalaena spp.)
- Harp seal (Phoca groenandica) and hooded seal (Cystophora cristata)
- Harbour porpoise (Phocoena phocoena) and Dall's porpoise (Phocoenoides dalli)
Breedings between these marine mammals near the North Pole are likely to result in fertile offspring, because many of these animals have the same number of chromosomes, said comment co-author Andrew Whiteley, a conservation geneticist at the University of Massachusetts, Amherst.