ERV has just put up an excellent post on why genetic diversity is important. She starts off:
Okay, so there are like 20,000 polar bears left.
Meh, who cares, right? I mean, there are still some. 1,600 plus the ones in zoos. 'Endangered' animals are fine! Yeah... No. Minor problem with decreasing population numbers: Its more than just the numbers. It's genetic diversity within those numbers. If those 1,600 pandas are all we have left, and those 1,600 pandas are genetically similar, they are in big trouble.
Easy example? Tasmanian Devils. While there are still 20,000-50,000 Tasmanian Devils left, they are being slaughtered by an infectious tumor. An infectious tumor that is taking advantage of the fact there is very little genetic diversity in the remaining Devil population.
Go over & read the rest - 20,000 polar bears may not be enough.