Gene Expression

The Out of Africa Replacement Model

The Multi-regional Model

The Introgression Model?

I have a question mark in the last case because this idea that we can have a simple to illustrate and verbally elegant "model" that we promulgate across the land might have to go. Parsimony to the back of the class...for now. John already chided me for presenting the old introgression via hybrid zone model as if it was the last word. I think it is important to not place evolutionary dynamics into a demographic box, the history of genes are reticulated, and do not respect our simple narratives. The old Out of Africa model was clean. An assumption of 1:1 correlation between mtDNA phylogenies and homonid demographic history was made which allowed the straight line inference that Africa is the locus of a recent demographic expansion which resulted in the replacement of all other human populations within the last 100,000 years. All mtDNA lineages are derived from African ones. But, all this says necessarily is that it seems that African mitochondrial lineages replaced all others, whether via demographics (e.g., population explosion or larger long term effective population size) or selection. The old Multi-regional model is anagenetic in that "lineages" are simply local subpopulations with particular ecotypes, but all of the various populations are interwoven by persistent gene flow, as well as periodic selective sweeps driven by advantageous alleles arising in subpopulations spreading. There is no lineage splitting.

A more nuanced model doesn't take an extreme position. For example, one can acknowledge that there was a recent demographic expansion out of Africa which characterized the rise of modern humans as we know them, while at the same time acknowledging that the local ecotypes might have persisted via introgression of locally selected alleles. How's that for a mouthful? The short of it is that Neandertals and other archaic species were on the ground in regional habitats for far longer than the expanding Africans. They possessed particular alleles which the Africans didn't have. Just like Europeans picked up maize and potatos from the New World populations, so Africans could have picked up MC1R & MCPH1 alleles which might come in handy. But just because Europeans adopted New World crops as their own does not mean that European culture is derived from New World culture. Similarly, just because local ecotypes were partially assimilated by the expanding African population base does not mean the African populations are now descended from the antecedent populations. Only some of their genes, and perhaps many essential aspects of their phenotype, might resemble the antecedant populations. But if it looks like a duck, and it quacks like a duck...well, you decide.