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Re: Livezey and Zusi's big bird morph analysis [...]
But why do you assume that the morphological characters shared by
grebes and loons *are* due to convergence? I agree with David here.
When it comes to assessing relationships between grebes and loons, why
is the convergence/homoplasy hypothesis better supported than the
common ancestry hypothesis? Why can't the characters associated with
foot-propelled diving also be potential synapomorphies?
Good question. The reason why I find it dubious is that the
foot-propelled diving characters in question don't actually align that
well between the two taxa on closer examination. The incorporation of
the femur into the body wall is accomplished in a different fashion in
the two clades, the swimming stroke is quite different between the two
(and produces very different stresses at the proximal limb), the
webbing differs very substantially, etc. In other words, if grebes and
loons are indeed sister taxa, then it appears (to me, at least) that
only a few characters related to semi-aquatic living would be
synapomorphies for the grebe/loon clade; most of their diving
specializations would still fall as convergent.
That said, I wouldn't be so skeptical if the analysis had resulted in
only one or two supposedly convergent groups falling together. What
really makes me raise an eyebrow is that Livezey and Zusi not only find
a grebe/loon clade, but also a owl/falconiform clade, a falconiform
clade including cathartids, a monophyletic pelecaniform clade, etc. It
could be that convergence is really that rare in neoaves, and that the
convergence/homoplasy hypothesis is incorrect in every one of those
incidences. Seems a bit unlikely, though.