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Random genetic drift yo!

People often talk about random genetic drift. Like sexual selection it is the deus ex machina of choice when you are shit out of luck in regards to hypotheses. And yet though it looms large in our minds R.A. Fisher dismissed it as an important evolutionary force. Sewall Wright in his Shifting Balance Model tended to emphasize the interactions of genes more than the random fluctuation of frequency as such. Drift only came into its own with the rise of Neutral Theory and molecular evolution. In any case, I’m going throw out an algebraic relation out there that I think is important to keep in mind when thinking of these issues.
Imagine if you will two alleles, variants, of a gene, p & q. p = 1 – q & q = 1 – p. The sampling variance moving from one generation to the next is defined by:
(p X q) / (2 X effective population), or (pq)/2N
The standard deviation of this value, is [(pq)/2N]0.5, its square root.
The key point is that as the population drops the sampling variance starts to shoot up, that is, drift to and fro from generation to generation. This is the root cause of the “random walk” process of changing frequencies and eventual fixation. Obviously a large population is less effected by this perturbation than a small one.

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