Many thanks to Michael Kubisch for this contribution.
An interesting recent article in Molecular Ecology asks the provocative question of whether cattle, sheep and goats are endangered species. ((Taberlet P, Valentini A, Rezaei HR, Naderi S, Pompanon F, Negrini R, Ajmone-Marsan P. (2008) Mol. Ecol. 17(1):275-84. Are cattle, sheep, and goats endangered species?)) Yes, that is species, not breeds. While nobody will question the fact that many livestock breeds all over the world are at risk of disappearing, the suggestion that whole species may be at risk would seem far-fetched.
Well, not if you’re a geneticist. The article nicely summarizes what’s been lost in terms of genetic variation and the overwhelming impression one gets is that it’s quite a bit. Even in breeds such as Holstein cattle where animals number in the millions, the effective population size, which is an indicator of the degree of genetic variation, would suggest that such populations tend to be relatively homogeneous. The primary reason for this is, as one might expect, mostly economic necessity. The selection pressure for increasing production of desired commodities has inevitably led to the loss of genetic diversity. What seems to have accelerated this loss is the use of modern technologies such as artificial insemination, which allows for rapid and widespread dispersal of the genetic attributes of relatively few males. And this may just be the beginning. The fact that advances in cloning technology now make it feasible to generate transgenic animals with added, deleted or altered genes means that such changes would by necessity depend on their dispersal on very few founder animals. So it is likely that this trend is not only continuing but may, in fact, be accelerating.
I was interested to see you writing about the point of the few males. I have been playing with the mathematics behind the genetics. We have two herds of pigs. In essence, over time we end up breeding the boar from the north to the boar from the south and many of the effects of the females vanish. It is almost literally as if we were breeding two males together. That was a startling side thought to the pencil pushing. All without any GMO techniques.