24 August, 2005

The strength of selection

PZ is lambasting Deepak Chopra today. Not for his New Age pseudo-Vedic/quantum mechanics mysticism spooge, but for his apparent anti-evolutionary stance. It's worth pointing out something that I think goes little appreciated by anyone, especially those idiots arguing against selection all the time, which is how powerful natural selection really is.

Here's some elementary population genetics: The basic meat of evolution is mutation followed by change in frequency of alleles. Let's say we're examining a particular position in the genome. Everyone in our species of interest has the same nucleotide at this position. Now, suppose one individual acquires a mutation here. If we have a breeding population of Ne diploid individuals, this means that mutation has a frequency of 1/2Ne. The probability of this mutation "fixing" in the population (that is, reaching 100% frequency) is:
Here s is the "selective coefficient", a measure of the deviation of the fitness of the individual with respect to the average fitness of the population. If the average individual produces n offspring, our mutant will produce n(1 + s) offspring. (Note that if s is zero, the mutation is selectively neutral and the fitness is the same.) For small s and large N, the above equation can be approximated as π = s.

What does this mean? Well, take our human population. We have a genome of about 3 billion bases. Typically this means about 100 new mutations per generation per individual. That is, your offspring will have a hundred completely novel changes to their genome. Most of the time this will mean nothing. Occasionally this will result in positively selected mutations. In a breeding population of 10,000 individuals (a relatively small population size, as our ancestors had), this provides millions of novel mutations every generation.

In other words, a novel mutation with even a small selective coefficient has a significant chance of being fixed. In the human population this means even mutations that produce marginal changes in fitness - a differential of 1001 to 1000 births per individual - will be fixed 0.1% of the time. A selective coefficient of 10-2 is powerfully strong selection. And it occurs completely invisibly to us. We would never note its effects; we produce far fewer than 100 offspring, and this tiny differential would never be observed without a complex, rigorous survey. But by the time a mere 10 individuals have acquired this mutation, its fixation probability has already climbed to 10%.

So remember this: natural selection is a powerful, efficient method for sculpting species, much stronger than our intuition suggests.

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