Chapter 5 of Gregory Cochran and Henry Harpending’s The 10,000 Year Explosion: How Civilization Accelerated Human Evolution (2009) is called “Gene flow,” and looks briefly at the evolutionary changes induced by mixing of genetically distinct populations.
The chapter begins with the concept of a gene allele “sweep,” or the manner in which a favourable gene variant can spread throughout a population of people where interbreeding is general, but not highly isolated and localised. While most gene mutations are harmful or neutral, some are beneficial (Cochran and Harpending 2009: 133). A particular gene can mutate into an allele that then creates a phenotypic trait in a living thing that happens to make that individual more successful and better adapted to its environment. That individual then usually has a selective advantage and survives to have more offspring, and in turn will tend to pass on the favourable allele to more offspring. Over time a small group of individuals with the new trait will survive and be more successful than others who lack it. Over time, a new trait will spread at large in a population and cause evolutionary change.
A gene “sweep” is the process by which a gene variant (an allele) can spread, over time, through a given population. A gene sweep may spread quickly over hundreds of years or, depending on population size and how well-mixed a given population is, over thousands of years. Agricultural populations often practice marriage between neighbouring villages or settlements, and even this local mixing – over enough time – can spread genes and traits over large populations (Cochran and Harpending 2009: 136).
Cochran and Harpending (2009: 137) use a model to suggest that a new allele that provides a selective advantage of 5% will – in a well-mixed population – rise to a high frequency in about 8,000 years.
However, population movements in history have also been a major vehicle by which genetic change and evolution have progressed. While geographical barriers (like oceans, deserts and mountains) and distance have certainly keep many human populations genetically isolated for long periods of time, as in the case of Native Americans and Australian Aborigines, in the Old World of Eurasia gene flow has been much more common.
As a result of trading, colonisation, conquests, the slave trade, and migrations, the Old World has seen a considerably greater degree of gene flow historically as compared with, say, the Americas before the late 15th century or Japan (Cochran and Harpending 2009: 144–145).
It follows, then, that such gene flow – given differential regional evolution – has also been an additional factor in driving genetic change in certain populations.
To take a concrete example, the Roman emperor Marcus Aurelius hired some 8,000 Sarmatians from what is now Southern Russia as mercenaries, and then sent them to Britain, where they appeared to have permanently settled. Cochran and Harpending (2009: 147) speculate that, if these Sarmatians introduced some favourable gene allele variants into Britain, over time many modern British people will now carry those alleles after a few thousand years of mixing. So even a small population movement can, over time, have large genetic effects.
Large-scale population movements and migrations have also been a more radical manner in which gene flow is accelerated. At the end of the Roman empire, there were large-scale migrations of northern Europeans into the Mediterranean world. One such group was the Vandals, who ended up in North Africa. Cochran and Harpending (2009: 151) speculate that the Vandals spread the allele that causes blue eyes into North Africa, so that the blue eyes now found amongst the North African Berbers and Tuaregs can be traced to gene flow between their ancestors and the ancient Vandals.
Cochran, Gregory and Henry Harpending. 2009. The 10,000 Year Explosion: How Civilization Accelerated Human Evolution. Basic Books, New York.