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There's been some discussion on digits and evolution, and I can't hesitate
to comment. Simple facts:
1) Reappearance of digital phalanges is not uncommon in evolution; look at
basically any recent relevant paper by Neil Shubin, David Wake, Pere
Alberch, Olivier Rieppel, etc., or if you're stuck on dinosaurs look at
iguanodontids (last time I checked).
2) New digits (whether they are homologous or not is another matter) have
appeared in ichthyosaurs (source: Padian lab's new postdoc, Ryosuke
Motani), to name one taxon.
3) Within populations, however, reversals in phalanx/digit numbers do
occur; e.g. the salamander _Taricha granulosa_, for example (Shubin, Wake,
and Crawford 1995; Evolution 49: 874-884), and plenty of others. It's clear
that the developmental potential may be there in many cases to generate
enough variation. If you've read any literature on evolution in the last
150 years or so, you know that variation is one of the raw components of
evolution. Like I've said before, if the developmental potential is there,
the evolutionary potential is there.
4) We know little/nothing of the ontogeny of most dinosaurs. Any statements
about their lack of developmental potential remain outside the realm of
empirical evidence. If we don't know their developmental potential,
statements like "digits don't re-evolve" might be true, but assuming them a
priori, and then turning around and using them as an argument against a
particular phylogenetic scenario, are asking for derision from anyone with
a shred of knowledge of development and evolution. Stick to the evidence.
5) Right, so you're thinking "If we don't know how dinos developed, then
how can we say they _did_ re-evolve digits?" We have to rely on ancillary
evidence, in this case a phylogenetic analysis. That's all we have, folks.
If we didn't have that, we couldn't say whether any dinosaur digits were
homologous or reversals or whatever.
6) If you want to argue about the phylogenetic analysis, fine, I'll stay
out of that. That's what the digit homology issue will ultimately stand or
fall on. But don't cite something like Dollo's Law as evidence for
anything; it may be a trend, but it's not been corroborated as inviolate,
and if you ask me it's already been falsified by numerous studies on extant
and extinct taxa.
Also see Shubin, Tabin, and Carroll's recent paper "Fossils, genes, and the
evolution of animal limbs" in Nature 388: 639-648 for a little review
reading of the current state of evolutionary developmental biology (please!
John R. Hutchinson
Department of Integrative Biology
3060 Valley Life Sciences Bldg.
University of California - Berkeley
Berkeley, CA 94720 - 3140
Phone: (510) 643-2109
Fax: (510) 642-1822