There was a paper published in 2008 that analyzed mitochondrial DNA from all 23 of the recognized "ecotypes" (groups shown to be each others closest relatives - not exactly the same as a subspecies). The authors found that bushbucks fall into two very distinct groups which were as different from each other as bushbucks are from other species of spiral-horned antelope. This is good evidence that there are actually two separate species of bushbuck. Further research and evidence is needed to be more confident but, for the time being, the authors propose the two species be designated
Tragelaphus scriptus (and called the kewel) and
Tragelaphus sylvaticus (or imbabala). The paper can be found at
http://www.wpazambia.com/Articles/mitochondrial DNA testing of bushbuck 2008.pdf. Some additional info is available at
Bushbuck - Wikipedia, the free encyclopedia.