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Re: emu history



David Marjanovic wrote:

I was surprised to see that he had mtDNA from five genera of moas, and that they cluster out quite reasonably, so the kiwi placement seems to be rather sound.

Alas, just because one part of a phylogenetic tree "makes sense", this does not necessarily mean that the entire tree is good. Some DNA sequences are better at capturing deeper divergences than others, depending on whether the constituent gene(s) evolve 'fast' or 'slow', and on the method of analysis used to crunch the data. Unfortunately, you cannot get the best of both worlds by combining 'fast' and 'slow' genes, and hoping the right tree comes out in the wash. (This is usually the intention behind sequencing WHOLE mitochondrial genomes, rather than relying on a small number of genes. This doesn't usually solve the problem.)


Thus, the recovery of a monophyletic moa clade in a given molecular study (a result that is strongly supported by morphological data) does not automatically mean that deeper branching events can be treated with equal confidence (even if they have strong bootstrap support). To give an analogous example, mitochondrial-DNA analyses of mammals typically recover most of the modern mammal 'orders' as monophyletic, but jump around at the base of the tree, making interordinal relationships difficult to interpret. For ratites, it may be that published mitochondrial-based analyses are giving us the shallow clades (moas; emu+cassowaries), but are misleading with regard to basal ratite relationships. In this respect, they may not be too different from anatomy-based phylogenies.



Tim

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