Reverse-genetic-drift and reverse-migration effects on estimates re ancient haplogroup percentages in local human populations

A principle of genetic history, is "genetic drift" (http://en.wikipedia.org/wiki/Genetic_drift ), meaning that in isolated populations, the y-haplogroups that are in a plurality or majority (supposedly) become more and more dominant in terms of percentage-wise representation in the population (soon perhaps I'll understand exactly how such occurs and be totally convinced of the validity of the concept).

So one could say that the opposite of genetic drift, would be haplogroups that dominate an area simply due to the area's isolaton, having been less dominant in terms of percentage representation in the given local population in the past.

At the same time it is reasonable to suspect that in the past, since localities were more isolated from each other, localities resembled each other less than they do now, in terms of percentage of persons in given localities possessing given haplogroup markers.

Extrapolating and projecting backwards in time accounting for such effects, even assuming very slight annual change in percentage representation of a haplogroups in localities over the past few thousands of years, the calculated level of change over the past few thousands of years from the distant past to the present, can be dramatic.

This compound-interest-type effect can be understood by toying with a compound interest calculator online.

Imagine two nations, land X, and land Y.
The following gives numbers and percentages for individuals carrying haplogroups A, B and C in 2000 AD for the two nations:

land X: A 20 (20%) B 10 (10%) C 70 (70%);

land Y: B 15 (15%) C 05 (5%) D 80 (80%);

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Now imagine that it is 1500 years BP (before present), in the year 500 AD. 1500 years ago due to the reverse migration effect as we go back in time, the B group had not migrated from land Y to land X, and the C group had not migrated from land X to land Y. As a result, the situation was:

(10 B's moves to Y, 5 C's moved to X in formula)

500 AD

X: a 20 (21%) c75 (79%).

Y: b 25 (24%) d80 (76%).

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Now imagine that it is 3000 BP, in the year 1000 BC. In the 1500 years transpiring between 1000 BC and 500 AD, genetic drift increased the percentage-wise dominance of the more common haplogroups in the localities. Therefore in 1000 BC the dominant groups were less dominant than they were in 500 AD. Thus in 1000 BC, the haplogroup percentage scores were closer to each other than they were in 500 AD:

1000 BC:

X: a 33 (35%) c 62 (65%)

Y: b 39 (37%) d 66 (63%)

In the end,

in land X, in terms of change in percentages from 2000 AD to 1000 BC, the change was: A 20->35; B, 10-> 0; C, 70-> 65;

And in land Y the change was: B, 15 -> 37; C, 5-> 0; D, 80->63.

Star of the show in terms of showing itself to have once been powerful in an area where it now just seems to be a small fry: haplogroup B, for whom the estimate of percentage representation in land Y using the formula is 40% for 1000 BC compared to 15% in 2000 AD.

Generally it seems possible that the effect of conceptual reverse-genetic-drift and reverse-migration when imaginatively going back in time, is that the estimate is that in the point back in time, the haplogroups that are weakly represented now in a locality were not represented at all, and the remaining haplogroups that are moderately represented now, were closer to other haplogroups in terms of percentage representation in the locality.


@2009 David Virgil Hobbs