The two animals look like variations on a theme: a large, harem-holding deer with branched antlers and a loud autumn call. Older classifications treated them as one species, Cervus elaphus, spread across the northern hemisphere.
Mitochondrial and nuclear studies published from the 1990s onward found the deep division falls not between Europe and America but between a western group, the red deer, and an eastern group running from Central Asia across Beringia into North America. The elk sits in the eastern group, and most authorities now recognise it as Cervus canadensis. The two still interbreed readily in captivity, which is why the split took so long to accept.
The naming remains a trap. In Europe, elk is the standard name for the moose, and the Shawnee-derived word wapiti was coined partly to avoid the confusion. How many species the genus Cervus should hold is still argued, with some treatments recognising a third Central Asian lineage.