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Re: Digits

Tracy Ford wrote (Re: Shubin et al.):

>It's an interesting article, but I have a problem with this being a
>caseof  "re-evolving" bones. Modern amphibians are a very plastic (I
>don't know if this is the right word) group of animals.

I agree wholeheartedly, and I think you'd agree that we know nothing of the
plasticity of dinosaur development either. We also don't know if the toes
apparently "re-evolved" in therizinosauroids were "the same," i.e.
homologous as digit I. We might assume it since they are similar, or maybe
it's just semantics and doesn't matter.

If we define homology by common ancestry, and diagnose it by specific
criteria (position, development, similarity, etc.), then our assessment of
homology still stands or falls based on our phylogenetic hypothesis. So
again, the argument about the feasibility of digital atavism is immaterial
given the lack of data on non-avian dinosaur development.

>I would believe that re-evolving toes, what ever, took place IF the
>later offspring showed these changes, but this is not the case.

Yes, it does seem that inheritance of digital atavisms is uncommon, and I
agree that it is a general trend that digits are not regained once lost.
But it is not an inviolate law; no developmentally minded evolutionist
would hazard to make such an assumption a priori.

More papers on the subject:
Alberch, P. 1985. Developmental constraints: Why St.Bernards often have an
extra digit and poodles never do. American Naturalist 126: 430-433.

Prentiss, C.W. 1906. Extra digits and digital reductions. Popular science
monthly 68: 335-348.

Muller, G.B. 1990. Developmental mechanisms at the origin of morphological
novelty: a side-effect hypothesis. pp. 99-130 in Nitecki, M.W. (ed.),
Evolutionary innovations.

                        John R. Hutchinson
                 Department of Integrative Biology
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