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# Effective population size

I didn't make a big point of it, but in our discussion of genetic drift so far we've assumed everything about populations that we assumed to derive the Hardy-Weinberg principle, and we've assumed that:

• We can model drift in a finite population as a result of sampling among haploid gametes rather than as a result of sampling among diploid genotypes. Since we're dealing with a finite population, this effectively means that the two gametes incorporated into an individual could have come from the same parent, i.e., self-fertilization occurs when there's random union of gametes in a finite, diploid population.

• Since we're sampling gametes rather than individuals, we're also implictly assuming that there aren't separate sexes.14

• The number of gametes any individual has represented in the next generation is a binomial random variable.15

• The population size is constant.

How do we deal with the fact that one or more of these conditions will be violated in just about any case we're interested in?16 One way would be to develop all the probability models that incorporate that complexity and try to solve them. That's nearly impossible, except through computer simulations. Another, and by far the most common approach, is to come up with a conversion formula that makes our actual population seem like the ``ideal'' population that we've been studying. That's exactly what effective population size is.

The effective size of a population is the size of an ideal population that has the same properties with respect to genetic drift as our actual population does.
What does that phrase ``same properties with respect to genetic drift'' mean? Well there are two ways it can be defined.17

Subsections

Next: Variance effective size Up: Genetic Drift Previous: Summary
Kent Holsinger 2015-01-25