Genetic drift plays a bigger role in evolutionary change when a population is small. In larger populations, the forces of genetic drift are muted. In a population of 1 million, one extra fast cheetah more or less isn't going to make much difference in the grand scheme of gene frequencies. In a population of 20, however, that single cheetah does make a difference — at least in terms of her genes — if she does (or doesn't) reproduce.
Consider the coin toss again. If you throw up a bunch of coins — say, 400 — the outcome would be lots of heads and lots of tails — about half of each, in fact. Although you'd be willing to accept a few more or less than half either way, you know that only a very small chance exists that all 400 coins will end up falling on the same side. Now imagine that you're tossing a small handful of coins — say, four. The possible outcomes are fairly limited: You could get two of each, three of one and one of the other, or four of either heads or tails. Although you're not likely to see the coins come up four of a kind, that result is still within the realm of possibility. In fact, if you compute the actual probability, you realize that, on average, one of every eight times you throw four coins in the air, they'll end up on the same side.
The point? That even though a handful of coins will on average land half heads and half tails, that's just the average result. Individual tosses will vary randomly, and the smaller the handful, the more likely the chance that the results will be much different from 50-50.
Now think about allele frequencies rather than head-tail frequencies. A population may have two alleles in equal proportion, 50 percent each, at one point in time. And then, at some later point in time, the frequencies are drastically different. The smaller the population, the more likely that this change is simply the result of random factors affecting which individuals did and didn't leave more descendents.
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