The future evolution of humans

I've written about some future predictions for human evolution before. A couple of readers sent me this article from Science (subscription required) by Michael Balter, titled "Are humans still evolving?" Here's a sample:

To many researchers, the answer is obvious. Human biology, like that of all other living organisms on Earth, is the result of natural selection and other evolutionary mechanisms. Some say the question itself betrays a misunderstanding of how evolution works. "The very notion that ... [ellipsis in original] we might not be evolving derives from a belief that all other life forms were merely stages on the way to the appearance of humans as the intended end point," says primatologist Mary Pavelka of the University of Calgary in Canada.
But other scientists point out that in developed countries, culture, technology, and especially medical advances have changed the evolutionary rules, from survival of the fittest to survival of nearly everyone. The result, they say, is a "relaxation" of the selective pressures that might have operated 50 or 100 years ago. "Biologically, humans are going nowhere," says anthropologist Ian Tattersall of the American Museum of Natural History in New York City. University College London geneticist Steven Jones agrees. "The central issue is what one means by 'evolving,'" Jones says. "Most people when they think of evolution mean natural selection, a change to a different or better adapted state. In that sense, in the developed world, human evolution has stopped" (Balter 2005:234).

Of course, this article has a purpose in asking the question -- otherwise, there would be no reason to cover it, since the question is a century old. The reason: lots of genes are showing evidence for recent evolutionary changes in humans. Some of these are obvious, such as various molecular defenses against malaria. Falciparum malaria in particular has affected human populations severely only for the past few thousand years, and its very high mortality rate has yielded a vast opportunity for genetic defenses to evolve -- even those, like the sickle cell trait, that have devastating side effects for homozygous individuals. Likewise, some populations have recently evolved the ability to produce lactase, the enzyme that digests lactose, longer into adulthood. This change is an adaptation to diets that contain significant undigested (by microbes) dairy products, and is most common in Northern Europe and North Africa.

Other changes are less obvious but nonetheless candidates for selection. The DRD4 gene has multiple alleles, one of which is associated with ADHD in Western societies. This allele apparently has been increasing rapidly in frequency over the past several thousand years, presumably due to selection on its behavioral effects.

The article takes these recent changes as a case in point that evolution is still happening in human populations. And of course it is correct within the bounds of the examples. The spread of HIV in Africa has likely resulted in strong selection on several molecular defenses, including the delta 32 CCR5 mutation which is currently at 13 percent in Europeans, probably because of past selection from smallpox. Other defenses are likely emerging but unknown so far.

But the potency of this example depends on two observations. First, HIV is so serious a cause of mortality in Africa (and indeed in many other regions of the world) today that the strength of selection resulting from it is high enough to be obvious. And second, mortality due to HIV is very likely to remain high for the foreseeable future -- long enough to constitute an observable trend.

For the most part, the kinds of selection that have been operating in the recent past cannot be shown to be effective today. How many people in Northern Europe are currently dying because of the inability to digest milk effectively as adults? How many Britons does smallpox kill each year? Even trends that currently exist in human health, such as the persistence of malaria and HIV in sub-Saharan Africa, are active targets of medical research and public health efforts. Much is made of the fact that medical science has reduced mortality manyfold from the traditional epidemic killers of large proportions of humankind. Even though this is indisputably true, mortality continues nonetheless, as does reduced fertility for many people. These facts mean that selection continues, particularly upon single-locus genetic disorders. The goal of continued research is to reduce the causes of early mortality, and restore lost fertility to those who want it. Those goals do not by themselves reduce the public health tragedies of today, but they make it very likely that today's maladies will be less severe in the future.

Medical technology has changed massively over the past 100 years, and there is every reason to expect that the change will be just as vast over the next century. This means that diseases that are incurable and unavoidable today will be cured or avoided in the future. It also means that people will likely have much more genetic knowledge about themselves in the future, and may be able to employ that knowledge to alter or manipulate the genes of their offspring in some way. In a previous post I wrote about the prospect of encoding information into genes as a sort of legacy project -- the genetic version of a vanity license plate. This kind of trivial change will join other more potent forms of genetic engineering. In other words, within the next century, the most powerful form of selection may be artificial selection (almost a technological version of meiotic drive) on the composition of genes themselves.

But the power of selection is not the only factor to consider. It is true that the power of natural selection on medical targets is decreasing. And it is likely that the power of artificial selection will greatly increase. But equally important to the prospects for the human species is the persistence of such selection as a trend. Here is where future prediction inevitably fails.

The potency of the article's examples of recent evolutionary changes is that these changes have formed substantial trends over the past several thousand years. If there were no trend, there would be no identifiable selective change, regardless of the strength of selection in any single generation.

If biomedical technology can change by light-years within a century, it endangers the prediction of any future trends. A hundred years is only four generations in humans, far less time than selection needs to have a substantial effect. Selection associated with HIV in Africa today is doubtless very high, but it is unlikely to last very many more generations before medical and economic breakthroughs reverse it. Selection is fast, in geological terms, but in social terms it runs like syrup.

On the subject of trends, we are on safer ground (although never completely so) to discuss evolutionary changes due not to selection, but instead to demography. Medical technology is not the only mode of rapid change in human societies. Social and technological changes have both combined to result in massive demographic shifts in Western countries. Consider that a hundred years ago the population of Europe and America was growing rapidly, with a fertility rate nearly double today's. Now, both populations have fertility rates below replacement, with the United States population growth supported by immigration, and Western Europe and Russia shrinking despite massive immigration. These demographic changes are the cause of strong evolutionary change in each of those regions, by migration and genetic drift, as alleles with a long history in Europe decline in frequency, and alleles from other regions of the world increase. These demographic shifts are almost certain to be much more powerful over the next century than any changes due to selection in those populations.

But selection will likely have a future effect that we cannot predict. There is approximately a one in a hundred million chance that any single nucleotide site will undergo a point mutation between a parent and a child. Across the genome, this means that you likely differ from your mother and father by fifty or a hundred new mutations.

In the preagricultural era, when there were only a few million people in the world, this rate of mutation means that only a small proportion of possible functional genetic changes occurred in any generation. For a population of 10 million, only around 1 in 10 possible sites had any new variants in anybody. Weakly selected changes would be unlikely to increase, even if they conferred a very slight advantage, because genetic drift would likely erase them before they reached an appreciable frequency. In other words, the evolutionary exploration of the genome was very limited. The human population could only move to those variants that were common enough to be selected and advantageous enough to overcome relatively strong genetic drift.

Today, with 6 billion humans, every one-off mutation from the human consensus genome sequence occurs in dozens of people. Many multiple-off mutations occur in some people. In a larger population, selection is more potent, because genetic drift is weaker. This means that the advantageous variants of the next fifty millennia are already appearing in the world today, and may inevitably be selected. The global population is exploring the entire mutational space, many times over, and novel mutations are no longer likely to disappear so rapidly due to genetic drift. Any near variants that confer an advantage are already on the way to fixation. Many of these may lose their advantage once biomedical technology catches up to them. But others will be more subtle, more difficult to market in pharmaceutical form, and these will slowly, steadily increase.

What are they? The level of selection necessary to drive a variant to fixation over 100,000 years or 200,000 years is very weak -- so weak that we will never measure it in a single generation, even if we assess the reproductive performance of thousands of people. So we don't know what those changes are, or which direction the trends will emerge.

Strong trends in selection are opposed by biomedical technology. Weak trends are unobservable. Which leaves us with, precisely nothing. How can we predict a future that is already happening? Is it any wonder people like me spend our time trying to predict the past?

References:

Balter M. 2005. Are humans still evolving? Science 309:234-237.