Genetic Chaos

Tuesday, June 22, 2004

Human demographic history: refining the recent African origin model

Recent studies of large portions of the human genome support a recent origin of modern humans from an African stock after a bottleneck of moderate size followed by a range expansion out of Africa. Under this simple scenario, patterns of molecular diversity suggest that balancing selection could be more prevalent than positive selection in coding regions.

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Microsatellite diversity and the demographic history of modern humans

We have examined differences in diversity at 60 microsatellite loci among human population samples from three major continental groups to evaluate the hypothesis of greater African diversity in this rapidly evolving class of loci. Application of a statistical test that assumes equal mutation rates at all loci fails to demonstrate differences in microsatellite diversity, while a randomization test that does not make this assumption finds that Africans have significantly greater microsatellite diversity (P < 10-8) than do Asians and Europeans. Greater African diversity is most apparent at loci with smaller overall variance in allele size, suggesting that the record of population history has been erased at repeat loci with higher mutation rates. A power analysis shows that only 35–40 microsatellites are needed to establish this difference statistically, demonstrating the considerable evolutionary information contained in these systems. On average, African populations have ~20% greater microsatellite diversity than do Asian and European populations. A comparison of continental diversity differences in microsatellites and mtDNA sequences suggests earlier demographic expansion of the ancestors of Africans.

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Microsatellite evolution in modern humans: a comparison of two data sets from the same populations

We genotyped 64 dinucleotide microsatellite repeats in individuals from populations that represent all inhabited continents. Microsatellite summary statistics are reported for these data, as well as for a data set that includes 28 out of 30 loci studied by Bowcock et al. (1994) in the same individuals. For both data sets, diversity statistics such as heterozygosity, number of alleles per locus, and number of private alleles per locus produced the highest values in Africans, intermediate values in Europeans and Asians, and low values in Americans. Evolutionary trees of populations based on genetic distances separated groups from different continents. Corresponding trees were topologically similar for the two data sets, with the exception that the (delta-mu)2 genetic distance reliably distinguished groups from different continents for the larger data set, but not for the smaller one. Consistent with our results from diversity statistics and from evolutionary trees, population growth statistics Sk and beta, which seem particularly useful for indicating recent and ancient population size changes, confirm a model of human evolution in which human populations expand in size and through space following the departure of a small group from Africa.

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The Geographic Distribution of Monoamine Oxidase Haplotypes Supports a Bottleneck During the Dispersion of Modern Humans from Africa

Every genetic locus mingles the information about the evolutionary history of the human species with the history of its own evolution. Therefore, to address the question of the origin of humans from a genetic point of view, evolutionary histories from many genetic loci have to be gathered and compared. We have studied two genes residing on the X chromosome encoding monoamine oxidases A and B (MAOA and MAOB). Both genes have been suggested to play a role in psychiatric and/or behavioral traits. To search for DNA variants of the MAO genes, the sequences of exonic and flanking intronic regions of these two genes were determined in a group of Swedish males. The sequence analysis revealed several novel polymorphisms in the MAO genes. Haplotypes containing high-frequency MAOA polymorphisms were constructed, and their frequencies were determined in additional samples from Caucasian, Asian, and African populations. We found two common haplotypes with similar frequencies in Caucasian and Asian populations. However, only one of them was also the most frequent haplotype in Africans, while the other haplotype was present in only one Kenyan male. This profound change in haplotype frequencies from Africans to non-Africans supports a possible bottleneck during the dispersion of modern humans from Africa.

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The four faces of Eve: hypothesis compatibility and human origins

Several different sources of evidence have been used to support a recent African origin for our species. It is commonly assumed that these sources of evidence support the same recent African origin. However, a close examination of the evidence available from four sources, including paleontology, archaeology, the level of human genetic variation, and the geographic structure of human genetic variation, shows that this is not the case. Each of these in effect supports a different recent African origin, and no hypothesis of a recent origin is compatible with more than two of them at a time. In contrast, all of these sources of evidence may be consistent with a multiregional model for our recent evolution.

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