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Wednesday, December 17, 2008

Recent evolution in humans

Did evolution stop once modern humans emerged in Africa? Or, to the contrary, did it speed up?

This question is addressed in the forthcoming book by Greg Cochran and Henry Harpending: The 10,000 Year Explosion. Harpending is an anthropogist and Cochran a physicist. Together they have produced a number of interesting research ideas in the area of human evolution (see below). I've read a pre-release draft of the book and recommend it highly. If you enjoyed Guns, Germs and Steel by Jared Diamond, then you owe it to yourself to read this book, which directly engages Diamond's thesis that geography (not DNA) is destiny.

I discussed research supporting accelerated recent human evolution by Cochran, Harpending and collaborators in an earlier post: We are all mutants now. The figure below is from a Times article by Nicholas Wade.

We are all mutants now: Some interesting new science suggests that human evolution has accelerated in the last tens of thousands of years. The study by Hawks, Wang, Cochran, Harpending and Moyzis (of UW Madison, Affymetrix, U Utah and UC Irvine) uses linkage disequilibrium tests on hapmap SNP data to determine that roughly 7% of all genes have undergone strong selection recently. The method looks for regions of DNA with similar SNP patterns. If an advantageous gene swept through a population in a relatively short time, replacing other variants, then the pattern of nucleotide polymorphisms in that area of the chromosome will be particularly uniform throughout the group. The results imply that we are all descended from mutants who, relatively recently, out-competed and replaced their contemporaries. The distribution of mutations is not uniform in different geographical populations (i.e., races). Recent evolution is causing genetic divergence, not convergence.

There is a good theoretical argument for why evolution may speed up due to population growth. Given a particular probability distribution for producing beneficial mutations, a large population implies a faster rate of incidence of such mutations. Because reproductive dynamics leads to exponential solutions (i.e., a slight increase in expected number of offspring compounds rapidly), the time required for an advantageous allele to sweep through a population only grows logarithmically with the population, while the rate of incidence grows linearly.

To elaborate on the last point, consider the set of mutations that are sufficiently advantageous that they would sweep through a population of N humans (i.e. reach fixation) in some specified period of time, such as 5000 years. If the probability of such a mutation is p, the rate of occurrence in the population is proportional to pN. Now imagine the population of the group increases to 100N. The rate of mutations is then much higher -- 100pN -- but the time necessary for fixation has only increased by the logarithm of 100 since selective advantage works exponentially: the population fraction with the mutant gene grows as exp( r t ), where r is the reproductive advantage and t is time. This rather obvious point -- that linear beats log -- suggests that the rate of evolution will speed up as population size increases. (A possible loophole is if the probability of mutations as a function of relative advantage is itself an exponential function, and falls off rapidly with increasing advantage.) If the Hawks et al. results are any guide, as many as 7% of all genes have been under intense selection in the last 10-50,000 years. (See here for another summary of the research with a nice illustration of how linkage disequilibrium arises due to favorable mutations.) Importantly, the variants that reached fixation over this period are different in different geographical regions.

Thus civilization, with its consequently larger populations supported by agriculture, enhanced rather than suppressed the rate of human evolution.

A related question is whether selection pressure remained strong after the development of civilization. Perhaps reproductive success became largely decoupled from genetic influences once humans became civilized? Not only is this implausible, but it seems to be directly contradicted by evidence. The graph below, based on English inheritance records, shows that the rich gradually out-reproduced the poor: the wealthy had more than twice as many surviving children as the poor. (Note the range of inheritances in the graph covers the middle class to moderately wealthy; the poor and very rich are not shown.) Thus, in this period of history wealth was a good proxy for reproductive success. Genes which were beneficial for the accrual of wealth (e.g., for intelligence, self-discipline, delayal of gratification, etc.) would have become more prevalent over time. In a simple population model, any lineage that remained consistently poor over a few hundred year period would contribute almost zero to today's population of Britons.

The graph is taken from this paper:

Survival of the Richest: The Malthusian Mechanism in Pre-Industrial England


Fundamental to the Malthusian model of pre-industrial society is the assumption that higher income increased reproductive success. Despite the seemingly inescapable logic of this model, its empirical support is weak. We examine the link between income and net fertility using data from wills on reproductive success, social status and income for England 1585–1638. We find that for this society, close to a Malthusian equilibrium, wealth robustly predicted reproductive success. The richest testators left twice as many children as the poorest. Consequently, in this static economy, social mobility was predominantly downwards. The result extends back to at least 1250 in England.

See also my review of Clark's A Farewell to Alms, and this video of a talk by Clark. When Clark wrote the book he wasn't sure whether it was genetic change or cultural change that led to the industrial revolution in England. In the video lecture he comments that he has since become convinced it was largely genetic. That doesn't jibe with the back of the envelope calculation I give below -- even in the optimistic case (largest effect) it would seem to take a thousand years to have a big shift in overall population characteristics.

Here's a very crude back of the envelope calculation: if, in a brutal Malthusian setting, the top 10% in wealth were to out-reproduce the average by 20% per generation, then after only 10 generations or so (say 2-300 years), essentially everyone in the population would trace their heritage in some way to this group. In our population the average IQ of the high income group is about +.5 SD relative to the average. If the heritability of IQ is .5, then in an ideal case we could see a selection-driven increase of +.25 SD every 2-300 years, or +1 SD per millenium. This is highly speculative, of course, and oversimplified, but it shows that there is (plausibly) no shortage of selection pressure to drive noticeable, even dramatic, change. If the estimate is too high by an order of magnitude (the rich group doesn't directly replace the others; there is inevitably a lot of intermarriage between descendants of the rich and non-rich), a change of +1 SD per 10,000 years would still be possible. There's clearly no shortage in genetic variation affecting intelligence: we see 1 SD variations not just within populations but commonly in individual families!

So where does this leave us?

1) The rate of positive mutations went up due to population growth. More importantly, the rate of mutations that were likely to sweep the entire population in a fixed period of time probably went up.

2) Natural selection did not abate: there is evidence for differential reproductive rates that are impacted by genes.

3) Humans living today are possibly quite different from our ancestors of 50,000 years ago. I would guess we are smarter and better suited to living in a complex society that requires cooperation and planning. We are also probably more likely to be lactose tolerant, nearsighted and bad at hunting ;-)

Cochran and Harpending's new book deserves wide attention and serious discussion.

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