Do large or small populations evolve quicker?

This isn't directly related to medicine, but it isn't exactly irrelevant either. It relates to the Hardy Weinberg equilibrium, but the logic behind it is interesting.

The answer is that a small population evolves quicker. Basically, this is because random events have a disproportionately large effect on shaping small populations, whereas big populations are largely insulated from its effects.

Let me explain by means of an example. Consider how rapidly a population could change if it only had 6 individuals, and the one with a rare gene (allele) fell off a cliff. The next generation would have 'evolved', as no one would have this gene anymore. Or consider the converse - in a population of 6 I hold a rare allele, but a freak lightning strike wipes out all the holders of the 'common' allele. Suddenly, the 'rare' allele becomes proportionately more common! This is an extreme (and a little bit silly) example of the phenomenon known as genetic drift - allele frequencies are changed by random chance, rather than by natural selection.

This effect is much harder to match with a large population - say of a million individuals. For a random event to wipe out, by sheer accident, all (or even most) of the holders of a rare allele is clearly much harder if here are tens of thousands of them. In other words, genetic drift has a smaller impact on a larger population. Therefore, the alleles in a small population tend to change more rapidly, which is to say that such a population evolves more quickly.

Furthermore, although it isn't often mentioned in textbooks, a mutation (as distinct from the random sampling error of genetic drift) will make its presence felt slightly more strongly, all things being equal, if the population is small (consider: a new allele out of six, or out of a million - the small population would show the effects more quickly).