Mule deer can be divided into two main groups: the mule deer (sensu stricto) and the black-tailed deer. The first group includes all subspecies, except O. h. columbianus and O. h. sitkensis, which are in the black-tailed deer group.[5] The two main groups have been treated as separate species, but they hybridize, and virtually all recent authorities treat the mule deer and black-tailed deer as conspecific.[1][5][6][7][9][10] Mule deer apparently evolved from the black-tailed deer.[9] Despite this, the mtDNA of the white-tailed deer and mule deer is similar, but differs from that of the black-tailed deer.[9] This may be the result of introgression, although hybrids between the mule deer and white-tailed deer are rare in the wild (apparently more common locally in West Texas), and the hybrid survival rate is low even in captivity.[8][9] Many claims of observations of wild hybrids are not legitimate, as identification based on external features is complicated.