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different genera (Bison and Bos). Recent studies of mtDNA (Janecek et al. 1996) suggest that bison and cattle are sufficiently closely related that they should be placed in the same genus—Bos—but that revision has not yet been accepted by the Nomenclature Committee of the American Society of Mammalogists, the body that sanctions such changes. Bison and cattle do have anatomic and physiologic similarities and are capable of interbreeding. They also are susceptible to similar diseases.
That bison and cattle are classified as separate species is less important than the length of time since they diverged in evolutionary history. That determines the period over which effects of natural selection, genetic drift, and, in the case of cattle, artificial selection accumulate: the longer the divergence time, the greater the expected differences between bison and cattle. The traditional view, based on paleontology and morphology, placed the diversion time at about 2 million years ago (McDonald 1981), but the more-precise DNA molecular-clock approach suggests that it is substantially greater. Examination of mtDNA control-region sequences (691 base pairs) revealed divergence of 0.09 (Bison bison versus Bos taurus) and 0.093 (Bison bison versus Bos indicus). Assuming a divergence over time of 2% per million years (Brown et al. 1979), those values imply an approximate time since divergence of 4.5 million years (J. Derr, Texas A&M University, pers. commun., 1997). That interval encompasses the evolutionary speciation of most of the currently recognized ungulate species (see, e.g., Georgiadis et al. 1990; Cronin 1991). Thus, although bison and cattle share many genes because of their common ancestry, each has been isolated for a long period during which independent mutation and selection could result in differences in physiology that equal or exceed those in morphology.
There is also differentiation below the species level, and two subspecies are ordinarily recognized (McDonald 1981): plains bison (Bison bison bison) and mountain or wood bison (B. b. athabaska). This distinction based on morphologic characters is supported by modern DNA analysis. Studies of bison at Elk Island National Park, Alberta, show that wood bison and plains bison are genetically distinct populations, based on genomic DNA restriction fragment length polymorphisms (Bork et al. 1991), and they are estimated to have diverged from a common stock around 5,000 years ago (Wilson 1969).
B.ABORTUSINFECTION AND TRANSMISSION
Much has been made of the difference in disease between bison and cattle. Certainly, B. abortus induces disease in bison and elk that differs from the