In popular fiction, mutants are cool. They have special powers and look like Hugh Jackman. In reality, though, mutations that have any effect at all are almost always bad for you. Everybody has a number of mutations of varying severity. Some completely destroy a gene’s function, while others cause lesser degrees of harm. Mutations occur every generation, while selection is always eliminating them. At any given moment, there will be a small percentage of defective copies of each gene in the population.
In a simple model, a given mutant has an equilibrium frequency μ/s, when μ is the mutation rate from good to bad alleles and s is the size of the selective disadvantage. To estimate the total impact of mutation at that locus, you multiply the frequency by the expected harm, s: which means that the fitness decrease (from effects at that locus) is just μ, the mutation rate. If we assume that these fitness effects are multiplicative, the total fitness decrease (also called ‘mutational load’) is approximately 1 – exp(-U), when U is where U=Σ2μ, the total number of new harmful mutations per diploid individual.
The mutation rate for any particular locus is low: for a typical gene, something like 10-5. But over the genome as a whole, the total rate is on the order of 1 per generation, or maybe a bit larger than that. Suppose U is 1: then the average person has a fitness that is less than two-thirds that of a mutation-free individual, one with all typos corrected. Some estimates have U as high as 4.2 in humans: in that case, average fitness is only about 1% of a mutation-free individual. This big change occurs because U is up in the exponent.
Suppose that some human population had their mutation rate increase by 50%? What would happen? They would immediately experience an increase in genetic disease. The first thing you would notice would be a higher rate of dominant genetic diseases like Marfan syndrome and neurofibromatosis. For the the case of a dominant lethal, the percentage increase in the first generation would be the same as the increase in the mutation rate – because dominant lethals must be generated fresh every generation. Less drastic mutations would increase more slowly, because there is already a substantial background level. Marfan syndrome decreases fitness by about 25% – it would take something like a century to reach the higher equilibrium rate. A mutation that decreased fitness by 1% would require about 2500 years.
Those 2500 years would make the Dark Ages look like a picnic. Things would get worse and worse. After a millenium or two, _nobody_ in this hypothetical population would be as fit as as today’s average. You would expect to see lots of changes, all bad. Lifespan would surely go down. Infant mortality and miscarriage rate would go up. IQ would decrease, probably more so than many other fitness traits, since more
than half of all genes are expressed in the brain. It’s a a big mutational target – more to go wrong. People would be crazier, too – as if we didn’t have enough trouble already. In the long run, thousands or tens of thousands of years, this population would adapt to the consequences of the higher mutation rate, assuming that it didn’t go extinct. Brains would cost just as much as they ever did, but would deliver less fitness per cubic centimeter: under these conditions the fitness-maximizing size would go down. Natural selection would also be reshaping other traits - you’d still be seeing adaptation to disease and climate and whatnot. But complex adaptations would not work as well.
Innovations, including useful innovations, are generated by a fairly small number of smarter-than-average people. That fraction would drop dramatically as average IQ decreased, because of the shape of the normal distribution, which decreases more and more rapidly with increasing distance from the mean. Innovation would probably stop. At the very least, the rate of innovation we’ve grown to love, fast enough to keep ahead of population growth, would falter. The world would become Malthusian. You’d see cats and dogs living together.
All this assumes reasonably random mating. It also assumes that the fitness-reducing effects of mutations are multiplicative, which seems reasonable but is not at all certain. It is possible to imagine ways in which certain patterns of selection could limit the increase in mutation load. For example, suppose that only a certain fraction of people could mate – say the top 30% in fitness – while others never reproduced. That would severely limit the increase in mutational load, since each genetic death would eliminate many bad alleles. This model, truncation selection, is halfway plausible in a dog-eat-dog Malthusian world, but nobody knows how close it is to reality. Geoffrey Miller would suggest that sexual selection would ameliorate this problem. Even if some of these optimistic scenarios are correct, people in our hypothetical high-mutation population would be worse off. The question is exactly how much.
What would a spelling-checked person, one with no genetic typos, be like? Since no such person has ever existed, we have to speculate. I figure that kind of guy would win the decathlon, steal your shirt and your girl – and you still couldn’t help liking him.