human migrations
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Migration is another important factor in human evolution that can profoundly affect genomic variation within a population.
Most populations are relatively isolated, however, although rare exchange of marriage partners between groups does occur.
An average of one immigrant per generation in a population is sufficient to keep drift partially in check and to avoid complete fixation of alleles.
Sometimes a whole population (or a fraction of it) migrates and settles elsewhere.
If the migrant group is initially small but subsequently expands, by chance alone the frequencies of alleles among the founders of the new population will differ from those of the original population and even more so from those among which it settles.
In this situation, group migration has an effect that in some respects is opposite to that of individual migration among neighboring populations: it creates more chances for drift and therefore divergence. The effect will be intergroup variation in allele frequencies.
Out of Africa
A recent synthesis of Y chromosome phylogeography, paleoanthropological and paleoclimatological evidence suggests a possible hypothesis for the evolution of human diversity.
Around 100 kya or shortly after, a small population of about 1,000 individuals (that is, a tribe), most probably from East Africa, expanded throughout much of Africa.
Then, between 60 and 40 kya there was a second expansion, most probably from a descendant population, into Asia and from there to the other continents.
This may be referred to as the ’standard model of modern human evolution’; it is also called ’out of Africa 2’ in recognition of an earlier expansion of Homo erectus from Africa into Eurasia around 1.7 million years ago and assumes that anatomically modern humans (also called Homo sapiens sapiens) replaced earlier poorly known species of Homo that descended from the first migrants of H. erectus.
Genetic data provide some indication that the spread of humans into Asia occurred through two routes. The first was a southern route, perhaps along the coast to south and southeast Asia, from where it bifurcated north and south.
In the south, these modern humans reached Oceania between 60 and 40 kya, whereas the northern expansion later reached China, Japan and eventually America (this might represent the second migration to America, associated with the NaDene languages, postulated by Greenberg).
The second was a central route through the Middle East, Arabia or Persia to central Asia, from where migration occurred in all directions reaching Europe, east and northeast Asia about 40 kya, after which the first and principal migration to America suggested by Greenberg occurred not later than 15 kya.
Books
The Great Human Diasporas: The History of Diversity and Evolution (Helix Books). by Luigi Luca Cavalli-Sforza, Francesco Cavalli-Sforza, Sarah Thorne. Paperback: 300 pages ; Dimensions (in inches): 0.86 x 9.21 x 6.13 Publisher: Addison Wesley Publishing Company; (October 1, 1996) ISBN: 0201442310 (Link to amazon.com)
References
Cavalli-Sforza LL, Feldman MW. The application of molecular genetic approaches to the study of human evolution. Nat Genet. 2003 Mar;33 Suppl:266-75. PMID: 12610536