
Interesting question RP.
Impala Aepyceros melampus are the only species in the tribe Aepycerotini and Aepyceros is the only species in this genus. There were up to 6 recognised subspecies at one stage, but recent molecular studies now recognise only two; The Common Impala A. m. melampus & The Black-faced Impala A. m. petersi. Some authorities also consider a third sub-species, A. M. rendilis or East African Impala as viable.
Southern Lechwe Kobus leche are members of the genus Kobus, with four recognised subspecies. K. l. anselli, K. l. kafuensis, K. l. leche & K. l. smithemani.
Impala are so different to other antelope that they certainly belong in their own tribe and as yet, there is no evidence of more than one species having ever been present. They are the only antelope that have the tufts of long black hair above the hooves of their hindlegs that cover the metatersal glands.
I retain the right to post my higgledy-piggledy Travel Tale in any way I deem fit.

