I recently watched the above lecture by Professor Steve Jones at Gresham College. This is my consolidated version and restatement Jones’s analysis for my own purposes. Like Professor Jones, I am inclined to answer the question, ‘Is human evolution over?’ in the affirmative, though based on a somewhat different basis.
In addition to changes to the natural environment, changes in social structure modulate evolution’s ability to influence how operates on Homo sapiens and its domesticates.
Society changes the environment
As the dominant mode of production changes, the specific types of food available can change significantly. For example, when human beings began to herd animals and then to develop horticulture and agriculture, the species they brought under their control were by no means the same as the species they had previously hunted and gathered. The range was much narrower, but also much more reliably available. So the number of calories consumed rose, but the range of nutrients, especially marginal items, shrank. Research makes it clear this had an ill effect on health as a whole, even while it allowed the overall population to grow. On the other hand, the concentrated and stable availability of, say, cattle allowed new forms of exploitation to develop. As well as the meat and hides and bones they had long taken from hunted animals, domestication allowed farmers to also extract milk. This was of limited immediate value, because the great majority of human adults at that time were lactose intolerant – milk made them sick. However, if it could only be digested, it would be a major addition to the farmers’ diet. And in some areas where herding was commonplace, this is what happened: in northwest Europe, west Africa and in certain areas of the Middle East, lactose intolerance declined. This was not a universal tendency: in many areas where cattle-herding was widespread, adult lactose intolerance remained the norm. However, it is most unlikely that adult lactose tolerance would ever have spread as it did had the core economic model not shifted as it did.
Natural selection relies on survival rates: if you do not live long enough to reproduce, you cease to exist from evolution’s point of view. On the other hand, the more children you have, the larger you potential impact on the future of the species. Historically speaking, human beings have had a highly variable survival rate. For example, in England in 1601 – the year Shakespeare completed Hamlet – only about a third of children lived to be 21. By the time Malthus was writing, it was more than a half. Now it is about ninety-nine percent. So differences in genetic heritage that would previously have been filtered out by early death have ceased to be a factor in human evolution, in modern England at least. And in fact in all areas of the globe, with the exception of sub-Saharan Africa, the post-War trend has been in the same direction.
Fertility
A second factor that social structure can have a profound impact on is differences in fertility – the likelihood that, regardless of whether or not you live to sexual maturity, you will actually reproduce. In many societies, a basic expression of inequality is that the powerful breed in excessive numbers and the less powerful if not actually wholly missing from future generations, are certainly less robustly represented. As Steve Jones expresses it, ‘if Genghis Khan was fertilising every woman in the landscape, lots of men were not doing it’.[1] Given the characteristic social imbalances of such times, the same was not true of women: Genghis Khan may well have had thousands of sexual partners (although ‘partner’ is probably unduly benign). So privileged men were much more likely to contribute to the human gene pool. For example, ‘There are probably a hundred million or more men in the world today who carry Genghis Khan’s Y chromosome’.[2]
By contrast, modern industrial societies (which is by no means lacking in inequality in narrowly economic or political terms) are demographically characterised by classes that have much more similar numbers of offspring, as do men and women. This is especially true of populations that have passed through the so-called ‘demographic transition’ to a modern population structure. So, the differential rates of reproduction onto which natural selection might latch are also being eliminated.
Bottlenecks and founders
A third evolutionary factor social development is rapidly eradicating is the bottleneck. Every new generation begins with a different population from its predecessors – different in number, distribution of the sexes and genetic makeup. These all have an obvious bearing on natural selection. For the most part, differences of this kind are either somewhat random or follow trends dictated by factors such as climate change. There are however moments in history when the basic numbers are radically reduced, and only tiny numbers of individuals, disproportionate numbers of particular groups or only a narrow selection of genes is able to continue. Then the evolutionary effect can be quite startling. However, the constraints on the population need to be very stringent (typically reducing the effective reproducing population to the dozens rather than the hundreds or thousands), but not so strict as to fall below the minimal viable population size.
For example, it has been estimated that the founding population size that crossed the Beringia land bridge and so populated the Americas numbered about seventy, or about one percent of the effective size of the estimated ancestral Asian population.[3] Among the ‘founder effects’ to which this leads are offspring who are substantially narrower and substantially different, genetically speaking, from the ancestral population of the founders themselves. And generally speaking it is well established that the farther a population is from our shared African origins, the less the genetic variation.
This is the result of countless bottlenecks of greater or lesser significance. The change that social development is imposing on evolution in relation to bottlenecks is simply to eliminate them. Aided by the power of modern transport, which can drop an individual on the other side of the planet the next day, the massive expansion and constant changeability of a global economy is eliminating barriers to the mixture of populations of all kinds. In short, the modern world has rendered the genetic bottleneck all but impossible.
Conclusion
These are by no means the only factors whereby human beings are derailing evolution. Social changes are largely unintentional. Technology is another obvious instrument for understanding and controlling the forces of variation and selection, not only ion ourselves but potentially in every organism or other material factor that impinges upon human biology. A good deal of potential evolutionary pressure is simply outrun by the speed of historical change. All in all, there can be no doubt that human beings are the products of evolution, but its can equally little be doubted that evolution is not going to have the last word on what it means to be human.
All diagrams and maps are taken from the transcript of Professor Jones’ lecture, which is available from Gresham College.
Notes and references
For detailed citations, click here.
[1] Jones (2015).
[2] Jones (2015).
[3] Hey (2005: 971).