Jason Rosenhouse and John Hawks have both commented on the introgression of cattle alleles into wild bison. J & J have hit many of the salient points, but let me suggest one issue: not all genetic loci are created equal. That is, "neutral" markers should be weighted less than "functional" markers. Of course, neutral markers probably aren't all neutral, and many functional markers are functionally relevant only in specified environmental contexts. The problem with these sort of questions is that I believe our "species concepts" are derived from gestalt psychology and our intuitive tendency to categorize "kinds" based on a few visible traits, and these few traits often derive their characteristics from a subset of genetic loci. The mapping of the words to the genetical reality of a flux of allele frequencies across populations is imperfect, and it is through the filter of these words that we elucidate our values about what is important. When we say "we need to maintain genetic diversity" what we really mean is that we wish to maintain correlation structure. But what does correlation structure of the vast sea of ancestrally informative neutral alleles really tell us? Is there some ancestral "bison essence" that is passed along neutral non-coding lineages? I say no! Cognitive psychologists, for example Paul Bloom, have been developing models which suggest that "innate dualism" and "essentialism" are prefab structures through which we comprehend our world, and so we have based our conservation policy from the bedrock of these intuitions. But if science is the ultimate arbiter then we need to reconsider the context of our values, because the full set of loci likely do not contribute to the "essence" which we seek to perpetuate. Alleles are more than the sum of their parts, consider the Africanized honey bee, they might be hybrids, but these "killer" bees have nasty traits which evoke less than a middling level of concern. Rather, particular behavorial tendencies inherited from their tropical forebears loom large, their docile European ancestry be damned! The blood does not tell I say, damn by phenotype and praise by phenotype!