Very interesting:

Biologists have until recently stated that Red Deer and Wapiti (or Elk) are the same species forming a continuous distribution throughout temperate Eurasia and North America, based on fertile hybrids that have been produced under captive conditions. Animal behaviour is generally different in captivity than in the wild, and the assumption that the same results would happen in the wild as in captivity is not necessarily the best test methodology to determine speciation.

Recent DNA studies conducted on hundreds of samples from Red Deer and Elk subspecies concluded, that that not more than 9 distinct subspecies of Red Deer exist and that they should be considered to be two separate species. The Wapiti or Elk from Northern and Eastern Asia and North America and the Red Deer from Europe, western Asia and North Africa represent two distinct species. Surprisingly the Elk is more closely related in DNA to the Sika Deer and to Thorold’s deer than to the Red Deer.