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Re: Thoughts on the biggest morphological bird analysis
David Marjanovic wrote:
Of course, but when different genes say the same thing, without an
identifiable selection pressure that could work on them all (like base
composition bias), I'll trust large molecular analyses over fossil-less or
otherwise too small morphological analyses any day of the week.
The thing is, the source of the homoplasy may not be immediately
identifiable. There may be (and often are) more subtle factors at work than
simple compositional bias. Compositional bias, if present at the DNA level,
might not be a product of any selection pressure (at least not for
vertebrates). Other biases work at the amino acid level, for example, and
are more insidious - and so they tend to be more difficult to identify.
Remember, the analysis is only looking for non-randomness in the dataset.
It has no way of differentiating the 'true' (historical) phylogenetic signal
from any biases (homoplasy) that are also present.
With morphological analyses, on the other hand, the putative homoplasy is
often plain to see. Like the grebe-loon clade, where foot-propelled diviing
is the prime suspect. Or the bat-cologo clade, where it's aerial
Thus, a bat-colugo clade is clearly out of the question, and a loon-grebe
clade looks unlikely -- here we have only three molecular analyses so far,
but we have Mayr's (admittedly small) morphological analysis that includes
the extinct flamingo diversity and *Juncitarsus*, while Livezey & Zusi only
use extant taxa in that part of the tree.
I agree with you. To some degree I'm playing devil's advocate here, because
many heavy-duty molecular analyses aren't as "case closed" as they're made
out to be. However, you can't rule out the possibility that a morphological
analysis with an expanded data set (i.e., more fossil taxa) might actually
confirm these clades, such as grebes and loons grouping together. As you
say, these hypotheses require further testing.
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