Re: "Common ancestor" in cladistics

[Nick Pharris <npharris@umich.edu>]

Quoting Dora Smith <villandra@austin.rr.com>:

> 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!
>
> I know part of it is that researchers assume that the greatest genetic
> differences took the most time to develop - but I'm not convinced that that
> actually tells us which group happened first if we have no ancestral DNA to
> compare it to.

Well, as with morphological evolution, we try to get information about the
ancestral state by comparing with one or more outgroups--taxa that are related
to the ones we're interested in, but not actually part of the group.  If you're
doing a phylogeny of placental mammals, you want to have one or more marsupials
in there as outgroups.

Unfortunately, there can be problems when the outgroup is only very distantly
related to the ingroup.  For birds, we have to rely on crocodilians, which,
since they are so distant from birds, can't tell us as much about the ancestral
avian genome as we would like.  Distant outgroups are known to cause problems
with rooting the tree (that is, deciding which ingroup taxon diverged
earliest).

Nick Pharris
Department of Linguistics
University of Michigan