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When less is more: founder flush

By razib on July 2, 2006.

A population bottleneck has a tendency of reducing the long term effective population size (harmonic mean) a great deal. I'm sure all of you knew that, and I'm sure you know that it also has a tendency of reducing variation. This is because low effective population sizes increase the power of genetic drift, which tends to expunge variation from a population.1

Great. The only thing though is that sometimes bottlenecks can "release" variation and make it available for selection. Additive genetic variance (heritability) increases as the genetic background is fixed on many loci, but not all, via a founder flush. I point this out just to suggest that sometimes the inferences drawn from population genetics can go against our expectation, because expectation in a probabilistic science must be modulated by variance.

1 - The approximate time until fixation of a neutral, that is totally dependent on drift, mutation is 1 X 4 X effective population. So, if you crank down the effective population new mutants will fix very quickly in populations and you have a situation where transient polymorphism is reduced.

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Not only that, small populations make it more probable that recessive traits will be expressed. If the result is advantageous, it then has a chance to spread through selection where it otherwise might not have.

sure, but i would think this is less important as recessive traits will spread eventually if they are beneficial. of course, it depends on your meta-population and mutational rate parameters (e.g., how substructured the overall pop. of a species/etc. is, how often the recessive, likely loss of function, occurs on a locus).

what i'm pointing to here is potential increase in quantitative variation, which i find very counter-intuitive....

Stuff to make your head spin...

Presumably *total* genetic variance ('broad' heritability) was reduced following the bottleneck, while additive variance ('narrow' heritability) increased. This is possible if non-additive variance (dominance, epistasis, assortative mating) is reduced by more than the additive variance is increased, but I wonder how typical this is. I note that the 'bottleneck' was very severe - the population reduced to only a few couples.