February 02, 2005

More on the multiregional model

The multiregional (MR) model of modern human origins, despite having been largely superseded by the Out-of-Africa model (OOA), just won't die, and why should it, given that much recent evidence seems to cast doubt on at least the pure OOA model which denies any contribution of older hominids in modern humans?

In a new paper in Human Biology, Dr. Relethford, a main proponent of MR shows under which conditions one of the main objections to MR can be overcome. Critics of MR start by the observation that our species is fairly homogeneous, or in genetic terms, has a very low effective population size of just 10,000. But, if we count as ancestors populations of erectus which spread over the earth more than a million years ago, then this number should be quite higher.

In this paper, the authors show that as long as these scattered human populations had (i) low gene flow among themselves, (ii) were subject to relatively frequent extinction events and (iii) migration between them was kin-structured, i.e., genetically related individuals tended to migrate, then the low effective population size could be explained, even though our prehistoric ancestors may have had much larger census sizes.

(i) must hold so as to prevent the formation of one big relatively homogeneous gene pool, (ii) would limit the time during which populations could exist and thus contribute to other neighboring populations, and (iii) would limit the amount of genetic variation exchanged due to migration, as family members are a more homogeneous unit than random samples of a population.

The authors are agnostic as to whether these conditions hold in our ancestral history, but in any case, their theory is worthy of consideration.

Human Biology 76.5 (2004) 689-709

Local Extinction and Recolonization, Species Effective Population Size, and Modern Human Origins

Elise Eller et al.


A primary objection from a population genetics perspective to a multiregional model of modern human origins is that the model posits a large census size, whereas genetic data suggest a small effective population size. The relationship between census size and effective size is complex, but arguments based on an island model of migration show that if the effective population size reflects the number of breeding individuals and the effects of population subdivision, then an effective population size of 10,000 is inconsistent with the census size of 500,000 to 1,000,000 that has been suggested by archeological evidence. However, these models have ignored the effects of population extinction and recolonization, which increase the expected variance among demes and reduce the inbreeding effective population size. Using models developed for population extinction and recolonization, we show that a large census size consistent with the multiregional model can be reconciled with an effective population size of 10,000, but genetic variation among demes must be high, reflecting low interdeme migration rates and a colonization process that involves a small number of colonists or kin-structured colonization. Ethnographic and archeological evidence is insufficient to determine whether such demographic conditions existed among Pleistocene human populations, and further work needs to be done. More realistic models that incorporate isolation by distance and heterogeneity in extinction rates and effective deme sizes also need to be developed. However, if true, a process of population extinction and recolonization has interesting implications for human demographic history.


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