 Reference Base  Conservation genetics of the black rhinoceros (Diceros bi... |
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World |
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Taxonomy |
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Black Rhino |
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The most widely accepted classification (Groves 1967) recognizes seven subspecies of Diceros bicornis, one of which D. b. Iadoensis, is probably extinct. Three other subspecies - D. b. brucii, found in Ethiopia and Somalia; D. b. longipes, which remains only in Cameroon and perhaps Chad; and D. b. cbobiensis; found in Angola - are down to a few dozen animals, if they are not already extinct (Western & Vigne 1985). D. b. bicornis, if it can be considered a distinct taxon at all, is found in Namibia and probably numbers less than 100 (Hall-Martin 1985; Du Toit et al. 1987). The remaining two subspecies, D. b. micbaeli and D. b. minor, or populations designed as such, will figure most importantly in the return of the black rhino, should this be accomplished. D. b. micbaeli, found in Kenya and Tanzania, has declined drastically as well, but still numbers between 500 and 1,000 and has increasingly received more protection in Kenya. The vast majority of the approximately 150 black rhinos in North American and European zoos are of this subspecies (Du Toit et al. 1987) and so will be important for future captive breeding efforts. D. b. minor is the most common remaining race, ranging from Kenya to South Africa, and with numbers at about 2,500, it is relatively the most secure.
In the absence of any clear morphometric differences, IUCN's African Elephant and Rhino Specialist Group has placed a high priority on genetic studies of black rhinos to resolve whether discrete populations could be identified (Du Toit et al. 1987). As a first step in applying molecular genetic techniques to questions of black rhino conservation, we have examined the mitochondrial DNA (mtDNA) of 23 black rhinos representing two morphologically defined subspecies and three geographic populations. We chose mtDNA because its rapid evolutionary rate has shown it to be a useful molecule for determining intraspecific relationships of many animals (e.g., Wilson et al. 1985; Avise & Lansman 1983). If the rhino populations surveyed here have had separate evolutionary histories for a considerable length of time, it should be reflected in the divergence of mtDNA's from animals in different populations.
The results of the mtDNA analysis strongly suggest a very close genetic relationship among all the black rhinos in our survey. Because of the generally rapid rate of mtDNA evolution in mammals, differences observed among rhino populations appear to indicate a very recent common ancestry. If mtDNA evolves at a rate of 2% per million years as suggested (Brown et al. 1979; Wilson et al. 1985), this common ancestry probably dates back no farther than 100,000 years. Indeed, the level of differentiation between the so-called subspecies is well within the range (0-4%) observed among members of other mammalian species (e.g., Avise & Lansman 1983), and even within the range (0-2% ) that has been observed among members of the same local population (Ashley & Wills 1987). Thus, there is no evidence from these data that the black rhinos we sampled represent 'evolutionarily distinct units.'
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