Meet my (probably mythical)1 ancestor, Marcus Antonius (83 BC - 30 BC), better known as Mark Antony. (Color photo not available.)
Mark Antony was a friend, and cousin, of Gaius Julius Caesar, although after Caesar's assassination he stopped praising Caesar. Mark Antony had a falling out with Octavian (Augustus) after the Second Triumvirate split up and he ended up in Egypt. The history is kind of interesting but not very relevant.
We're mostly concerned about Mark Antony's genes. Near the end of his life he had three children by Cleopatra, Queen of Egypt; the twins Alexander Helios & Cleopatra Selene II and Ptolemy Philadelphus. This led to gene flow between the Italians and subpopulations in the Middle East. (There were other liaisons that contributed to gene flow in both directions between the Middle East and Europe.)
Before moving to Egypt, Mark had several wives in Rome. One of them was Octavia Major and they had a daughter, Antonia Minor. Antonia married Nero Claudius Drusus Germanicus and one of her sons was Claudius Cæsar (Tiberius Claudius Nero Germanicus), Emperor of Rome. At the time Antonia and Drusus were living in Lugdunum (Lyon, France).
Claudius married Valeria Messalina (granddaughter of Octavia) and their daughter was Genvissa (Venus Julia). Claudius then married Julia Agripina and had more children, including Emperor Nero. We aren't interested in Julia Agripina except to note her scientific contribution to the understanding of eukarotic transcription [Mushrooms for Dinner].
Genvisa married Arviragus, King of Siluria, in 45 AD. Siluria was a kingdom in the south of Wales and at the time they were resisting Roman occupation. Arviragus became King of the Britons. Their son was Meric (Marius) , King of the Britons. Do you see where this is headed?
Meric married Penardun and their son was Coel I "Old King", King of Siluria. Coel had a SON who in turn had an unnamed GRANDSON who had a daughter named Alofe (Aife).
Alofe married Fiachadh III Streabhruine, 120th Ard Righ of Ireland, and their son was Muirreadhach Tireach , King of Connought, 122nd Ard Righ of Ireland.
Muirreadhach married Murien and their son was Eochaidh Moihmeodhain (Echu Mugmedón), 124th Ard Righ of Ireland.
Eochaidh married Carthan Casduff and their son was Niall Nóigiallach - Niall of the Nine Hostages, my ancestor and the ancestor of about 100 million other people.
Thus, Marcus Antonius is also my ancestor.
The figure below show that the various subpopulations within Europe are genetically distinct (Novembre et al. 2008). See my recent posting, Genes and Geography with a link to Razib's explanation of the structure of populations at Gene Expression [Genetic map of Europe; genes vary as a function of distance].
This data represents only a small percentage of the genetic variation in Europeans. Much of the remaining variation does not show a geographic distribution like the one in the figure because the variants arose much earlier. They have had time to spread to all subpopulations, or perhaps they pre-date the founding of the European population.
Novembre et al. also had to restrict their analysis to those 1,387 individuals who had both sets of grandparents from the same region. Many of the remaining group of 1,805 individuals did not know where their grandparents were born but a substantial number had grandparents from two different regions. What this means is that there is substantial ongoing gene flow between the various subpopulations
What does this have to do with Mark Antony? Quite a bit, actually. Looking at the figure from the Nature paper one can't help but be struck by what it says about population structure and gene flow in the past. The pink individuals in the upper left-hand have clearly been partially isolated from the rest of the European population for quite some time—equivalent to about 40-60 generations.
We know that this group has received alleles from Italy via Mark Antony and Niall of the Nine Hostages and probably from a great many other Italians who were living in Roman Britain. This level of gene flow amounts to just a trickle and the foreign alleles might easily be diluted out by random genetic drift.
We can think of gene flow in the opposite direction by considering what might have happened if a favorable allele had arisen in the Irish population about 1500 years ago. While it might have spread rapidly in Ireland, chance are it would not have made much impression in the rest of the European subpopulations until very recently. All bets are off now that humans have become so mobile but it is worth keeping in mind that the populations of most other species probably look a lot like ours did only a few centuries ago.
New beneficial alleles will not make much headway in 2000 years because gene flow between subpopulations is very low. There's no reason to assume that it was any different in the ancient past—it may even have been worse. Think about that the next time you hear about some hypothetical allele that arose 50,000 years ago and became fixed in the entire species. That's not very likely.
1. See comments. It looks like Genvissa, the presumed daughter of Claudius, is a mythical character made up many centuries after her presumed marriage to the King of the Siluria.
Novembre J, Johnson T, Bryc K, Kutalik Z, Boyko AR, Auton A, Indap A, King KS, Bergmann S, Nelson MR, Stephens M, Bustamante CD (2008) Genes mirror geography within Europe. Nature, Published online 31 August 2008 [doi:10.1038/nature07331]