Over at West Hunter, Greg Cochran has been introducing a a fairly new and potentially important theory of the genetic origins of race differences in IQ. It's less a theory of evolution than of devolution. The mechanism causing effective differences, he argues, is less selection for higher IQ due to differences in the environment (e.g., winter versus tropics selecting for forethought); instead, a large driver is differential rates in random mutation leading to differences in average level of deleterious genetic load, which tend to correlate with climate warmth.
What would happen if people moved somewhere where the mutation rate was far lower?
Their genetic load would decrease with time, assuming that they were still subject to much selection. Today, everybody has hundreds of nicked or broken genes: selection keeps eliminating them, while mutation keeps creating them. The suspicion is that their effect is quite large. This hypothetical population would have fewer and fewer. In a few thousand years, they would lose most of the variants that decrease fitness by 1% or more.
Cochran's next post looks at some data on the rates at which random mutations creep into the reproduction process.
Posted on July 14, 2012 by gcochran9
Several recent papers give me the impression that there is regional variation in mutational load. One can slice this a number of ways. Dan MacArthur and company looked for mutations that knocked out genes – loss-of-function or LOF mutations. Mutational load is the sum of all deleterious mutations – LOF mutations are a clear-cut subset of total mutational load.
Some of the LOF mutations they are found are common, and are presumably neutral, maybe even beneficial, but most are rare and likely deleterious. The kicker is that they found significantly more LOF mutations in their African population sample than in their European and East Asian samples – 25% higher. That was unexpected.
Population history (and mutation rate) determine the variation you expect to find in neutral genes, but significantly deleterious mutations should be in mutation-selection balance. A neutral variant might easily be a million years old, but a deleterious variant will last, on average, 1/s generations, when s is the decrease in fitness caused by that variant. A mutation that decreases fitness by 1% should disappear in 100 generations or so, about 2500 years. Ancient bottlenecks should not influence the frequency of such noticeably deleterious mutations.
Another related paper, by Jacob Tenessen et al, looked at a large set of coding genes, sequencing many times (average depth of 111x) for high accuracy. As in in MacArthur’s paper they found that the average person carries many probably-deleterious mutations, mutations which are individually rare. Each person carried, on average, mutations expected to change function (almost always for the worse, although usually only a little for the worse) in 313 genes (out of the 15,585 they studied.
They looked at African-Americans and Americans of European descent, about a thousand of each. They saw what MacArthur’s group did: there were significantly more probably-deleterious mutations in the 80%-African population. When they used a loose definition of functional variation, about 20% more : with a more conservative definition, which should have a higher fraction of truly deleterious genes, about 29% more.
... The only simple explanation (that I can think of) is a higher mutation rate.
One possibility is that heat tends to cause a higher mutation rate.
Henry Harpending then summed up:
Posted on July 16, 2012 by harpend=
The model that Greg is dancing around suggests (1) that there is variation in mutation rate dependent on temperature or something correlated with temperature, (2) higher mutation rates cause a higher genetic burden in human populations, (3) leading to IQ reduction and other minor dings
Here's my model of this theory (which is probably pretty woozy):
Imagine, say, a factory that builds a complex product, such as a car, according to a complicated set of instructions. But, the instructions on how to build the next car are passed on via the Game of Telephone, with mistakes inevitably creeping in. Sometimes, big mistakes are made, and the resulting car is such a disaster that it can't function at all and has to be scrapped. But, most of the individual mistakes are minor and just mean, say, that instead of delivering 268 horsepower, the engine generates 267. Over time, the Telephone Game build up mistakes until a car is completely unusable and has to be scrapped. At that point the workers go find a better car and get the instructions for that car relayed to them. So, on average, most cars don't come off the assembly line performing at spec, but they perform well enough to make it through a test drive.
Now imagine two factories making the same car from roughly the same overall design. One is in Nagoya and the other in Lagos. It's so hot and humid alla the time in Nigeria, unfortunately, that the workers get distracted during their Game of Telephone and have a higher rate of errors when transmitting plans from one generation to the next.
In the comments, commenter extraordinaire Jason Malloy writes:
While not fully or explicitly articulated, this is the first New Big Theory of race differences in quite a while, and an interesting alternative to the reigning sociobiological models available since the 1980s. In the latter models intelligence and reproductive differences are seen as consequences of natural selection in divergent latitudes, but this new model replaces natural selection with accumulated mutational burdens. The differences at lower latitudes are not selectively advantageous, but dysfunctional.
Dr. Cochran notes that complex adaptive systems, involving the functioning of many genes, should be the most vulnerable to genetic load, so this would obviously be the brain and probably reproductive physiology. So in addition to higher general mortality, dysfunctions associated with mutational burdens might include:
- Lower intelligence
- Higher retardation
- Higher mental illness
- Lower birthweight
- Higher premature births
- Higher infertility
- Higher reproductive deformities
- Higher miscarriage (and general obstetric complications)
- Lower sperm quality
Of course there is a difference between establishing population differences in genetic load, and proving that this is related to population differences in socially valued traits. I’m not sold on this as a replacement for sociobiological models, although there are aspects that make it useful and attractive in different ways. For example, I recently found that ethnic differences in rate of homosexuality are inversely correlated with latitude. Since theories of selectively advantageous homosexuality fall flat, this theory seemed like a better fit.
In the comments to Henry's post, I offer a couple of tentative criticisms, which you can read there.