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Re: "Common ancestor" in cladistics


Dora Smith wrote:

Methodologically we rarely have DNA from any common ancestors at all, so
what is done is to chart or measure genetic differences between existing
groups, on the theory that the groups with the most genetic differences are
teh most distantly related.

For the most part this holds for eukaryotes (including ourselves and other vertebrates). However, it breaks down completely for prokaryotes (bacteria and archaea) because of the rampant exchange ('swapping') of genes, sometimes between distantly related organisms. (And if it wasn't for this promiscuous gene-swapping, eukaryotes would never have evolved in the first place.)

Ribosomal RNA sequences are usually used as the datasets when investigate the relatedness of bacterial and archaeal 'species'. When we construct a phylogeny based on ribosomal sequences of bacteria or archaea, we are not recovering a phylogeny of the actual species, but of a solitary gene. By contrast, when we investigate relationships within a group of vertebrates or plants or arthropods, we can be fairly sure that the phylogeny of a certain gene does reflect the phylogeny of its owner. (Of course, there are important exceptions, since multicellular organisms may acquire genes from elsewhere.)

For instance, we know that rheiformes (like ostriches) and a closely related
group are most distantly related to all other living birds, that chickens,
geese and ducks are next most related to all other groups of living birds,
and somehow passeriformes end up at the other end of the spectrum. Some
researchers find rheiformes to be the oldest group, some find chickens,
geese and ducks to be the oldest group, and some actually insist that
passeriformes are the oldest group!

By 'Rheiformes' you probably mean Palaeognathae (ratites and tinamous); but I take your point. Many phylogenies have the Palaeognathae branching off first, followed by the Galloanserae (chickens, geese, ducks, &c), then all other birds, with the Passeriformes the last clade to branch off. The idea that passeriforms are the oldest group of modern birds was based on one or two molecular studies, and never had much support; newer analyses appear to have killed off this idea.



Tim

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