No! Basic understanding. Understanding not even of genetics but of probability.
This is the "official" continuation of a discussion TerryT and I have been involved in, lately totally off-topic-ly in this other thread. Terry said:
Abracadabra, bottleneck, drift, founder effect. With such magic words anything becomes possible.
... diversification indicates no bottlenecks, founder effect or drift, very unlikely in a population confined to just one region.
What I mean to prove here is that diversification in typical Paleolithic conditions (very low population density, relatively frequent colonization of new lands, specially in the late MP and early UP of Eurasia), necesarily implies drift and founder effect. Bottlenecks instead are a non-normal extreme event that might or might not have happened.
For that I will call as witness the source of all knowldge: God - err... I mean: Wikipedia.
1. Drift: In population genetics, genetic drift (or more precisely allelic drift) is the evolutionary process of change in the allele frequencies (or gene frequencies) of a population from one generation to the next due to the phenomena of probability in which purely chance events determine which alleles (variants of a gene) within a reproductive population will be carried forward while others disappear.
By definition, genetic drift has no preferred direction, but due to the volatility stochastic processes create in small reproducing populations, there is a tendency within small populations towards homozygosity of a particular allele, such that over time the allele will either disappear or become universal throughout the population.
This eventual outcome of drift, when an allele displaces all others within a population, is normally refered to as fixation. Fixation may also happen because of non-random processes such as selection.
2. Founder effect: In population genetics, the founder effect refers to the loss of genetic variation when a new colony is established by a very small number of individuals from a larger population.
Illustration of different possible founder effects from a simplified hypothetical population with two alleles
In addition to founder effects, the new population is often a very small population and so shows increased sensitivity to genetic drift, an increase in inbreeding, and relatively low genetic variation.
I say (and I think most will agree) that these two elements were fundamental in Paleolithic genetics, very specially in the colonization of Eurasia and related lands (Sahul, America). They are no abracadabra: just business as usual.
One could argue that, after Neolithic, the effects of drift have become negligible - and that's certainly the case of modern huge and highly intercommunicated populations. But that was not the case in the Paleolithic, when small bands of hunter-gatherers made up that human reality. Therefore understanding these concepts is essential to understand the formation of Humankind, very specially on light of haploid genetics.
The third concept that Terry likes to mix with the others like if these were the same thing is bottleneck. It is clearly not the same: a bottleneck is a extreme (not normal) phenomenon, where a population is not decimated but almost annihilated.
A population bottleneck (or genetic bottleneck) is an evolutionary event in which a significant percentage of a population or species is killed or otherwise prevented from reproducing, and the population is reduced by 50% or more, often by several orders of magnitude.
This is a very rare happening, though founder effects can appear like population bottlenecks because their effects are similar.
Population bottlenecks have been detected apparently in Eurasians (but not in Africans), slightly sharper among East Asians than among Europeans. This may indicate a real bottleneck (near-extinction) for instance after an event of massive influence like the Toba explosion but it is also possible that the apparent bottlenecks in Eurasians mean just founder effects.
Caveat: I may have misrepresented Terry's views in this post. Please read the comments for his explanations. Most importantly:
As I hope I've made obvious elsewhere I totally accept drift and founder effect (as well as bottlenecks to varying extents) have been important during our evolution. What I am totally uncomfortable with is where they are invoked to explain away inconvenient facts that conflict with particular theories about our evolution.