Population and diversity.

A little while ago, I left you all with this graph, and I asked which had more diversity, and why.
The answer is `Caribou,` because for each loci, there's a wider range of possible values (or `alleles` as we call them in the biz). While diversity in µsats (these chunks of DNA) doesn't translate 1:1 into total diversity, they're often a good proxy for them. Take Muskox, for example: They're monomorphic (meaning they have no diversity) at many regions of neutral diversity. Additionally, they're monomorphic at regions that are important to the function of the animal - such as MHC, which is important for disease resistence in us critters with backbones.

Part of the reason why Caribou have so much more diversity than Moose is probably due to the sheer number of critters - there's so many more Caribou than there are reindeer. Why, taken together, The Western Arctic Herd and the Mulchatna Herd of caribou outnumber the humans in the state of Alaska! That's not a trivial number of animals, and that's before you add in other major herds. Moose, to contrast, probably number between 60 and 80,000 animals in the state, depending on who's doing the counting. They exist at a pretty low density overall.

Why does population matter? Well, there's two reasons why. And I'll use two anologies.
First, imagine you have ten marbels, each a different colour. You have them all on the ground, when you drop a hammer and smash one accidentally. One of your colours is now no longer represented in your pool of marbels. But if you have 100 marbels, with 10 of each colour, and you drop your hammer, you could smash many more marbels before you lose any colours. The same is true with animal diversity - the more animals you have, the less likely you are to accidentally kill off all the animals with one sort of `allele.`

Second, imagine you're running a poor-man's xerox place. Instead of machines, you've hired a bunch of teenagers to copy things by hand. Teenagers are flakey, and make mistakes while copying pages. Right now, you're xeroxing stuff for the national archives, so all their mistakes will be preserved as they're made. If 1 in 10 teens makes a mistake, then on average, your shop of 10 teens will make one mistake each job. But if you're running a larger shop of 100 smelly, moody teenagers a) you're to be pittied and b) you'll, on average, make 10 mistakes per job.
The same is true with animals. When DNA is replicated so it can be passed on to baby animals, we invariably make mistakes in copying it. Some of it is bad, most of it doesn't matter, and a little fraction of the mistakes are good - this leads to some of the variation we see in nature. Each human, for example, carry around about 400 new `copy mistakes` that most other humans don't have. The more animals you have, the more new forms of `alleles` will be formed through copy mistakes.