|Genghis Khan. Unknown artist.|
A technical paper published a few days ago in the European Journal of Human Genetics shows how, at least in humans, genes can increase in frequency in a population not necessarily because they represent an adaptive advantage in their own right but because they are bound to other replicators, such as cultural memes, which give them an advantage. This linkage may be entirely due to chance.
For example, a culture which is hierarchical and expansionist, and especially where sons inherit the power, authority and privilege of their fathers, may facilitate the spread of genes carried by powerful men, especially where power gives access to females and comes with a higher standard of living so children are more likely to survive. The actual genes benefiting from this may be completely unremarkable.
This paper found that it was possible to identify a number of clusters of particular haplotype of the Y-chromosome (only carried by males and so indicating the male inheritance line). By counting the number of mutations in a given region of the Y-chromosome and assuming a regular mutation rate, it was possible to estimate the time when this variant arose in human evolutionary history. By assuming that the geographical location where most diversity was found was close to the place where the variant originated it was possible to estimate a likely place where this variant arose.
Source: Wikipedia - Genghis Khan. Created by User Astrokey44, modified by Sting
The secret to getting your Y-chromosome into future generations is, of course, to have lots of sons, and for those sons to have lots of sons, preferably for three or more generations, and what better way to do that than to be a powerful tribal leader? Genghis Khan reputedly had an organised band whose job it was to keep him supplied with several young women a night and reputedly fathered hundreds of children. His sons by his official wife, Jochi, Chagatai, Ögedei and Tolui all became powerful rulers of khanates after the death of Genghis.
No doubt his many 'unofficial' sons would have been high status too. His four sons by his Empress Börte all had several sons many of whom also became powerful rulers. Of Tolui's many sons, Möngke became Great Khan of the Mongol Empire, Kublai became the first Yuan Dynasty Emeror of China as well as Great Khan of the Mongols and Hulagu become the first Ilkhan of Persia.
Not surprisingly then, 8% of men in 16 populations in Asia and 0.5% of men worldwide have Genghis's Y-chromosome. It's difficult to imagine any other Asian ruler of that time able to command the social and economic power to produce as many surviving and powerful male descendants as Genghis Khan so there can be few serious contenders for the progenitor of this particular Y-chromosome cluster. It's always possible that Genghis inherited his unique Y-chromosome from his father or grandfather and that the key mutation didn't arise in the sperm that contained his Y-chromosome but it is surely to Genghis, his sons and grandsons that credit must go for its disproportionate success rate.
In the case of Giocangga, it is estimated that he had about 1.5 million descendants in 2005, mostly in and around northern China and Mongolia where this Y-chromosome cluster is found, whereas it is almost completely absent in Han Chinese. The Qinq dynasty was founded by upstart Manchurian tribal warlords from the north, ruled for many generations and the emperors normally had many concubines. Again, we see several generations of powerful men with many sons, many of whom were also powerful men in their own right and a political process which continued this for several generations.
Of his great-great-grandsons one was Colm Cille, or St Columba who took Christianity to Scotland, converting the Picts and facilitating a union of the Gaelic and Pictish tribes to found modern Scotland. Another great-great-grandson was Saint Máel Ruba. One of the last Gaels to play a major role on Irish history was Hugh O'Neil, Earl of Tyrone, on whose confiscated lands following the 'Flight of the Earls' from Ireland, the Ulster Protestant Plantations were founded. The rebellion Hugh O'Neil led was the last attempt by the native Irish Gaels to defeat the English and regain Irish independence. Hugh O'Neill was a direct descendant of Niall Noígíallach.
Irish geneticists have estimated that 21% of men in north-western Ireland, 8% from the rest of Ireland and 2% of New York men carry Niall's Y-chromosome. Chances are that if you're an O'Neil or one of its variants, you do to.
What we have in all these situations is an ability to produce a lot of male offspring who also produce a lot of male offspring - the classical recipe for evolutionary success, in this case of the 'male' chromosome. But in these cases, of course, the thing that allowed these males to produce lots of males and so to give their Y-chromosome an advantage over other Y-chromosomes, had little or nothing to do with genes. It was almost all to do with the other major replicators which play a large part in human evolution - cultural memes. It's probably no coincidence that a lot of the estimated times these clusters originated and the places they originated in, are in areas where political and economic development would have produced a hierarchical society.
So we see here an example of how memetic culture can form an incidental alliance with genes and cause these genes to become much more numerous in the genepool for reasons having little or nothing to do with the genes themselves. Human cultural hierarchies, inherited power, and the access to females that that often brings about, are of course the biological equivalent of the alpha or dominant male with a harem such as we see in many herding and social species. It may be that some genes are involved in this ability to dominate but often, especially with humans, the main factor might be who your father was rather than what genes you got from him or your mother, and especially what your culture is if that favours inherited power and deference to authority. There will be many other genes riding piggyback on any genes involved just as there will be many riding piggyback on the cultural memes.