In the last post I talked about the importance of genetic diversity in helping a species survive environmental changes and disease. Today I want to say a little about why genetic diversity in farm animals is threatened.
The first reason is that large-scale industrial agriculture tends to favor what it considers the most "productive" breeds. For shepherds, this means multiple births for the largest number of lambs, fast early growth for the largest possible market lambs, and big white fleeces that are easy for large mills to process. Thus commercial agriculture leaves behind many breeds, breeds that are smaller, slower to mature or that produce multi-coated or colored wool.
The second factor is globalization. The now-global nature of industrial agriculture has meant the world-wide spread of favored commercial breeds. Unfortunately, this has meant that many local breeds have fallen out of favor, even if they have developed over time to fit perfectly into their native environment. For example, the Red Maasai sheep native to Kenya, which are well-suited to arid conditions and resistant to internal parasites, were the predominant breed in Kenya until the 1970s. Since then, though, their existence has been threatened by the importation of larger, less drought-tolerant and less pest-resistant Dorper sheep from South Africa.
Finally, technologies like artificial insemination have made it possible for a single animal to father literally hundreds of thousands of offspring. Globalization insures that these half siblings can spread out all over the world. Here the most striking example I've found comes from the world of dairy cattle. According to the Canadian Farm Animal Genetic Resources Foundation, the popular Holstein dairy bull, Hanoverhill Starbuck, fathered over 200,000 daughters around the world between 1985 and 1996. Although Starbuck died in 1998, his clone, Starbuck II was born in 2000, insuring that Starbuck's genes continue to spread.
Here's a handy infographic that sums these issues up for easy reference: