Anthropology.net

One more from Molecular Biology and Evolution, this time investigating whether or not diversity seen in 280 bases of mtDNA from 886 people from 27 indigenous Americans is localized amongst populations. Or in other words, is the diversity seen in this short segment of mtDNA random or unique to groups?

The paper, “Native South American Genetic Structure and Prehistory Inferred from Hierarchical Modeling of mtDNA,” tells us in fact,

“[the] main discovery is that Eastern South America harbors more genetic variation than has been recognized. We find no evidence that there is increased identity by descent in the East relative to the total for South America. By contrast, we discovered that populations in the Western region, as a group, harbor more identity by descent than has been previously recognized, despite the fact that average identity by descent within groups is lower. In this light, there is no need to postulate separate founding populations for the East and the West because the variability in the East could serve as a source for the Western gene pools.“

Pretty cool how all it takes is a short amount of mtDNA to consider who founded whom.

Lewis, C.M., Long, J.C. (2008). Native South American Genetic Structure and Prehistory Inferred from Hierarchical Modeling of mtDNA. Molecular Biology and Evolution, 25(3), 478-486. DOI: 10.1093/molbev/msm225

About two weeks ago, I blogged on how variation mtDNA was used to reconstruct an idea of Pleistocene population growth. That study was very remarkable because illuminated a large 5 fold increase in South Asian populations about 50,000 years ago. But we haven’t been able to use polymorphisms in autosomal loci to the same. A new paper in Molecular Biology and Evolution tries to figure out a way to overcome this limitation, by testing to see whether or not mitochondrial and Y chromosomal DNA show similarities in the spread of polymorphisms in populations that have experienced a recent increase in effective population size.

The paper, “Contrasting Signatures of Population Growth for Mitochondrial DNA and Y Chromosomes among Human Populations in Africa,” honed in a large sequence from the Y-chromosome and the cytochrome c gene in the mtDNA from 172 males from 5 African populations. Four different statistical tests were applied to test for population expansion: Fu’s Fs statistic, the R2 statistic, coalescent simulations, and the mismatch distribution.

The authors find,

“…Patterns of mtDNA polymorphism better fit a model of constant population size for food-gathering populations and a model of population expansion for food-producing populations. In contrast, none of the tests reveal evidence of Y chromosome growth for either food-gatherers or food-producers. The distinct mtDNA and Y chromosome polymorphism patterns most likely reflect sex-biased demographic processes in the recent history of African populations. We hypothesize that males experienced smaller effective population sizes and/or lower rates of migration during the Bantu expansion, which occurred over the last 5,000 years.”

Pilkington, M.M., Wilder, J.A., Mendez, F.L., Cox, M.P., Woerner, A., Angui, T., Kingan, S., Mobasher, Z., Batini, C., Destro-Bisol, G., Soodyall, H., Strassmann, B.I., Hammer, M.F. (2008). Contrasting Signatures of Population Growth for Mitochondrial DNA and Y Chromosomes among Human Populations in Africa. Molecular Biology and Evolution, 25(3), 517-525. DOI: 10.1093/molbev/msm279

The creative PNAS study that Razib pointed out the other day is out, and I’ve read it. It comes from two biologists at Stanford, Deborah Rogers and Paul Ehrlich, who studied the canoe design of 10 Polynesian groups and one Fijian group for functionality and symbolism. The paper, “Natural selection and cultural rates of change,” is open access, so please download a copy and read what the authors have to say first hand. They decided on these Oceanic cultures because they were colonized by one cultural group that radiated and became relatively isolated from one another. In other words, little outside influence, or noise, from other cultures has theoretically impacted Oceanic canoe design.

They find that functionality (traits that affect whether or not the occupants of the canoe will survive or not) changes very little, whereas symbolism in canoe design (such as aesthetic, spiritual, and decorative) changes much faster. From this observation, they conclude that “cultural change, like genetic evolution, can follow theoretically derived patterns,” and that natural selection is the driving force behind the evolution of cultural traits. The authors rationalize boats are being selected naturally through a quote from the French philosopher Alain,

“Every boat is copied from another boat… Let’s reason as follows in the manner of Darwin. It is clear that a very badly made boat will end up at the bottom after one or two voyages and thus never be copied… One could then say, with complete rigor, that it is the sea herself who fashions the boats, choosing those which function and destroying the others.”

This quote doesn’t make much sense to me. It sure is a simplified analogy… one that is supposed to help us digest and reduce… But I’m left a bit confused on how the authors found evidence of natural selection out observing intelligent boat design. Unlike boat designers, who learn and modify boat design based upon past experiences, natural selection does not operat with conscious understanding of trial and error. Natural selection in nature is independent. Boat design is not independent. Boat designers can go study past designs, learn from their mistakes, and make modifications… So is not functional boat design just selection?

That’s one flaw I see in this paper, one that I haven’t really completely thought out. I do appreciate how the authors are one of the first, in my knowledge, to attempt to integrate how ‘cultural things’, such as items that serve a life or death purpose in everyday life, have a functional constraint. Often we’re taught that ‘cultural things’ are arbitrary but we know we want our things to work, and we’ve will always design them to work better.

I’ll leave you with words from, Nina Jablonski, an anthropologist that I really respect and whose work I appreciate immensely. She christened this paper as, “one of the most significant papers to be written in anthropology in the last 20 years.” Man, that’s a pretty amazing endorsement to have, but just cause Jablonski casted her vote doesn’t mean I’m sold, nor does this make it into the trophy cabinet for classical anthropology papers.

Rogers, D.S., Ehrlich, P.R. (2008). Natural selection and cultural rates of change. Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.0711802105