Steve isn’t sure that variance of mutational load1 betweein individuals is responsible for important differences in beauty. Well, no doubt it isn’t responsible for the whole range of aesthetic countenance, but I would be curious to see how much it accounts for differences within a range of siblings.
Here is the equation for mutation-selection balance of deleterious recessives:
In other words, the equilibrium frequency of q (the presumptive non-wild type) is ~ the square root of the mutation rate divided by the selection coefficient. Here is the derivation.
Obviously for a single locus the frequency for a potentionally lethal allele is rather low within the population, but consider the circumstance where you have a trait where the developmental stability and expression is tightly controlled by a wide range of loci. Remove lethality of a homozygote and assume additivity (so that deleterious mutations are not masked by a “good copy”). Complex organisms like human beings are a literal flux of bubbling mutational froth against which selection barely keeps up.
I have mentioned before that evolutionary psychologist Geoffrey Miller has suggested that the human brain is so overdeveloped because it serves as a signal for mutational load, so many genes contribute to the cognitive phenotype that the character of the mind is a reasonable proxy for the overall health of the genome. Of course physical beauty is another assessment of genetic health, as only individuals with low a mutational load can devote the resources to the expression of a gaudy and aesthetically extravagant phenotype.
This is all an excuse to post two pictures of Jessica Alba below the fold.
Related: If you are curious, check out the faddish epiphenomenon that is “Runaway” Sexual Selection.
1 – I was sloppy with definitions and used the term “genetic load” before when I was speaking in the individual context when this term is usually used in a populational context. Sorry.
Posted by razib at 06:28 PM