Genetic drift, Hardy-Weinberg and molecular evidence

Last time

Before we start: last time, in Variation, the t-test and natural selection, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Chance, not choice

Natural selection is not the only thing that changes a population's alleles. Sometimes chance alone does it, especially in small groups. And to know that any change is happening, you need a baseline: what the allele frequencies would be if nothing was changing. That baseline is the Hardy Weinberg principle.

Genetic drift

Genetic drift is a random change in allele frequencies from one generation to the next. It is caused by chance in which individuals breed and which alleles pass into the gametes. It has a much bigger effect in small populations. In a very small group, a rare allele can easily be lost completely, or become fixed, by chance. Unlike natural selection, drift does not favour the fittest. An allele can spread even if it gives no advantage, or even a disadvantage.

The founder effect and bottlenecks

The founder effect happens when a few individuals leave a large population to start a new one. The new population has only a small sample of the original alleles. So allele frequencies in the new population can differ from the original, and rare alleles may become common. Rare genetic diseases are more common in some isolated communities for this reason. A bottleneck is a sudden fall in population size, from disease or disaster. The survivors carry only some alleles, so genetic diversity falls.