Did the Wayne and Jenks article compare other canid sequences with
the jackals'? Past hybridization in one population could introduce
disparate mtDNA sequences.
Similarly, other populations of black-backed jackals need checked
to ensure that multiple mtDNA lineages were not present at the origin of
the species. If the ancestral population had two very different mtDNA
sequences, then one sequence may have been lost in one pack and the other
in the other. Also, perhaps "black-backed jackals" actually are two
Finally, even if black-backed jackals have unusual rates of
mutation in mtDNA, it still may be relatively constant in other lineages.
There's no a priori reason why mutations should occur at a constant rate;
each case needs checked against the fossil record.
Any "molecular clock" needs tested within the taxonomic group under
consideration before any credibility should be given to it. I don't know
if it's the high ratio of molecular biologists to paleontologists, but
molecular-clock based dates get far too much credibility relative to the
fossil record. Inclusion of error bars for the dates would help.