Re: [asa] How big a deal is homology?

From: Terry M. Gray <>
Date: Wed Mar 28 2007 - 12:34:55 EDT

I'm with James here in the "reality" of taxonomic similarities/
differences. For the most part (except perhaps in the fossil record)
it's not hard to tell the difference between a reptile and a mammal
or a hymenoptera and a diptera. The classification hierarchy is, in
my opinion, the first and foremost evidence for evolution.
Evolutionary theory explains it and I think it's what compelled
Darwin and the early evolutionists to propose the theory. Many
dispute this, of course, including, anachronistically, Linnaeus
himself. But at least it can be said that evolutionary theory is a
powerful plausible explanation for taxonomic relatedness. Also, my
support for this argument does not mean that I deny that there are
borderline or disputed classifications based on traditional
morphological characteristics. However, those are "anomalies" that
don't really undermine the force of the explanation as a whole.

However, the breaks that make for morphological taxonomy aren't
really there for molecular taxonomies. But the taxonomies match up
for the most part. This, I think, is the remarkable INDEPENDENT
evidence for evolution (and the fact that there is a huge temporal
gap between the two data sets). This is not to say that there aren't
issues with molecular (or morphological) taxonomic data.
There are questions of parsimony, possible horizontal gene transfer,
low probability interbreeding events, retroviral activity, etc. that
complicate molecular taxonomies. But, interestingly, further
molecular taxonomic studies can help sort out these anomalies. (And,
interesting, such things (except for parsimony) may actually be the
stuff of "macroevolution".) I was intrigued by the posting yesterday
about the marmoset mosaics/hybrids. How was it all sorted out in the
end? The answer: further sequence comparisons. Basically, molecular
evolutionary studies.

By the way, where does the idea of horizontal gene transfer come
from? It comes from genetic marker analysis (and nowadays sequence
analysis). How do we know it's horizontal gene transfer? Well,
because the sequence in question (or the marker in question, say with
McClintock's original maize transposon research) doesn't belong
there. How do we know it doesn't belong there? Further sequence

Some, Walter Remine, Cornelius Hunter, etc., call this hopeless
circular (or at best creating epicycle after epicycle). I think it's
normal science and even good science, especially now that mechanism
for such horizontal gene transfer events are observed and fairly well-


On Mar 27, 2007, at 7:40 PM, James Mahaffy wrote:

>>>> Brent Foster <> 03/27/07 4:08 PM >>>
> Yes that was my point actually. I accept the argument that a common
> designer would use common designs. But I was thinking more along the
> lines of your examples Terry. We know that DNA sequences can be
> inherited. And genetic markers are used to establish identity, family
> lines etc. I don't think too many YECs had a problem with the DNA
> evidence against O.J. If the same genetic markers are used to
> establish
> phylogenetic relationships between related species, I don't see how
> one
> can except one and not the other. To bring in the Micro- vs.
> Macro-evolution contrast, there seems to be a continuum between family
> groupings, racial groupings, species groupings, genus groupings etc.
> ****** me ********
> Woa!!!! In at least Zoology and angiosperm taxonomy there seem to be
> real breaks (you can argue continuum on some) but the breaks seem
> to me
> who is now teaching Zoology real and must be dealt with. One of the
> problems is that a monphylogentic system tends to de-emphasise them as
> much or more than a classical system that tended to emphasize groups.
> In its very philosophical foundations cladism assumes all (or at least
> almost all variation occurs with in a monophyletic line). In case you
> wonder I am not TE either or ID for that matter.
> The divisions between them are arbitrary.
> ****** me ********
> No they really aren't. Morphological taxonomy worked quite well and
> still is fairly important.
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Terry M. Gray, Ph.D.
Computer Support Scientist
Chemistry Department
Colorado State University
Fort Collins, CO 80523
(o) 970-491-7003 (f) 970-491-1801

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Received on Wed Mar 28 12:35:18 2007

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