Genetic canalization is the extent to which an organism is buffered against the effects of mutations. Waddington said “developmental reactions, as they occur in organisms submitted to natural selection…are adjusted so as to bring about one definite end-result regardless of minor variations in conditions during the course of the reaction”. Canalization can act to buffer against environmental perturbations, and selection for resistance to such environmental noise may also produce resistance to genetic noise. But right now I’m thinking about genetic canalization.

Up to some point, the effects of not too many, not too serious mutations would be buffered: those mutations wouldn’t change the phenotype. In the same way, your typical tractor is not designed to nanometer tolerances: parts can be somewhat out of spec – up to some limit – without messing up performance. And that tractor is better than one that did require nanometer tolerances, which we couldn’t manufacture. A genotype that had high fitness, but only when everything is perfect, no mutations, could on average be competitively inferior to one that had lower peak fitness, but in which that fitness didn’t decline rapidly with increasing small-effect genetic load – a fault-tolerant design, a fitness plateau rather than a sharp peak. This has been called ‘survival of the flattest’.

Remember when I talked about compensatory mutations in small populations making up for increased load from inefficient selection? This is the just the long run of that trend: eventually compensatory mutations develop into canalization mechanisms.

Canalization is a product of natural selection. There would be stronger selection for efficient canalization in a species with more genetic load: in particular, in a species that over the long haul tended to have fairly low population size. So you might expect to see stronger genetic canalization in big carnivores than, say, mice. And you would probably see it in the great apes and hominids as well: I don’t think that we were ever numerous until recently. Surely primitive, Oldowan-type hunter gatherers had lower population density than later, sophisticated hunter-gatherers, who had much improved methods. And those later, sophisticated hunter-gatherers weren’t exactly overflowing in numbers themselves.

So, maybe people have strong canalization. It could be that this let Neanderthals keep the show on the road for hundreds of thousands of years even though their effective population size was small. And we know that they did keep the show on the road: theory suggests that anatomically modern humans should have walked right over them on first encounter (last interglacial) – but that didn’t happen.

It might explain why load doesn’t seem to have much effect on IQ over most of the range, why we haven’t seen general IQ depression in the children of old men,