The role mtDNA plays in the evolutionary differentiation of species

Bob Koons shares his reflections on a Biogeography class discussion from some weeks ago. I've been unable to upload many student submissions to this blog due to severe tendonitis which limited my computer use for some time, but I'm beginning to catch up and hope to have the remaining student submissions uploaded before the semester ends!

Neotropical diversification of montane populations have been studied mainly in the lowlands of geographical regions simply due to the fact that there is a much larger diversity of species inhabiting the lowlands compared to the highlands. Studies use multiple approaches of statistical methods to get their data to agree. Which test is the best one depends upon the question being asked?

Species can be traced back to their origins using mtDNA, which is directly passed down from the mother. A species that has genetic ties to an ancestor and all its descendant taxa is called monophyletic and a species that has an ancestral taxon and some, but not all, of its descendant taxa; an artificial taxon. (Lomolino et al 2006) Geography plays a role in how a species diversifies over time as was demonstrated by the Finch scenario. The finches migrated south from Mexico to the Northern Andes Mountains of South America. Some finches traversed the Andes and migrated south along the east side of the Andes. The geographic isolation separating the two groups allows for natural selection to run its course allowing for the distinct differences we see in the finches today.

So does this apply to humans also? Can human mtDNA show this geographical divergence differentiating our species? An article entitled, “Geographic origin of human mitochondrial DNA: Accommodating phylogenetic uncertainty and model comparison” by John Huelsenbeck and Nikita Imennov compare the two main theories of the origin of the humans, which are the “out of Africa” and the “regional continuity hypothesis. The “out of Africa” hypothesis relates that modern humans all came from a common ancestor in Africa. Humans spread from there to all parts of the globe, and subsequently wiping out other “homos” that were encountered. The “regional continuity hypothesis” says a single species of homo originated throughout the old world connected through gene flow, after any migration from Africa occurred. Statistics using mtDNA show that the out of Africa scenario is the most possible origin for modern humans. A Bayesian inference statistical analysis was used to accommodate phylogenetic uncertainty from all trees then comparing the probabilities. Bayesian statistics is defined as, “Of or pertaining to statistical methods that regard parameters of a population as random variables having known probability distributions.”In their article Huelsenbeck and Imennov 2002 state, “In this study, we point out how Bayesian inference can be used to accommodate phylogenetic uncertainty when comparing five different models for the origin of modern human mtDNA.” 200 sequences of mtDNA were analyzed, each sequence was 428 sites long, from the hypervariable region I (HVRI) of the mitochondrial region of modern humans and an out group sequence from Neanderthal mtDNA. They used 40 sequences from each geographical region of the globe in their analysis; Africa, Europe, Asia, Americas, and Australia were represented. As discussed in class, and in this paper, a statistical analysis can be modified in many ways. This study used a uniform prior on all possible trees, another way to analyze the data would be to perform a coalescence prior, or to chose another method reconstructing the ancestral geographic area of human mtDNA. Another way of statistical modifications could include a stochastic two-stage model or a coalescence process with different populations connected by variable levels of migration. Still other ways exist that can modify an analysis; the trees could be reconstructed using a molecular clock restraint and incorporated into a Bayesian or maximum likelihood framework. (Huelsenbeck and Imennov 2002) A DNA sequence from Neanderthals may give a clearer answer to the out of Africa hypothesis. MtDNA can be useful to determine human geographic origin after all. Whew! From this we can clearly see that data can and is modified in many different ways to suit a desired outcome.

References:

Huelsenbeck, J. and Imennov, N., 2002. Geographic origin of human mitochondrial DNA: Accommodating phylogenetic uncertainty and model comparison. In Systematic Biology Vol. 51, No 1 pp.155-165.

Lomolino, M.V et al. 2006. Biogeography. MA: Sinauer Associates.

Dictionary.com, Bayesian Statistics. Retrieved October 5,2008.