Introduction

So far in this course we've talked about changes in genotype and allele frequencies as if they were completely deterministic. Given the current allele frequencies and viabilities, for example, we wrote down an equation describing how they will change from one generation to the next:

$$p' = \frac{p^2w_{11} + pqw_{12}}{\bar w} \quad .$$

Notice that in writing this equation, we're claiming that we can predict the allele frequency in the next generation without error. But suppose the population is small, say 10 diploid individuals, and our prediction is that $p' = 0.5$. Then just as we wouldn't be surprised if we flipped a coin 20 times and got 12 heads, we shouldn't be surprised if we found that $p' = 0.6$. The difference between what we expect ($p' = 0.5$) and what we observe ($p' = 0.6$) can be chalked up to statistical sampling error. That sampling error is the cause of (or just another name for) genetic drift - the tendency for allele frequencies to change from one generation to the next in a finite population even if there is no selection.