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Re: Measuring Morphological Distance



Mike Taylor (mike@tecc.co.uk) wrote:

<To which I got a lot of answers largely to the effect that it was a
stupid questions (it's a fair cop :-) because the groupings (genus,
family, order, etc.) are all so arbitrary.  But as I was thinking through
this recently, it did occur to me to wonder whether anyone has ever
proposed -- or even implemented -- some objective method of measuring how
different two specimens are.>

  Just to note, Sibley and Alquist offered a table of genetic distance
(percentage of code that agreed) to determine various determinations for
use of names, and specifically, ranks. Thus, 2-5% of the code was a
subtribe, and got the -ina suffix, and this increased relatively, absed on
their studies of avian genetics. This was offered to arbitraily offers
which were what rank. I have not seen the method applied.

<Now of course, there are all sorts of problems: incompleteness of
specimens is one, but even assuming good specimens -- say, the complete
skeletons of _Gorgosausus libratus_, represented by USNM 12814, and
_Coelophysis bauri_, represented by one of the Ghost Ranch skeletons -- is
there a way of saying "These two animals at 74.5% morphologically
similar"?>

  There is a very problematic issue to taking an approach on this matter.
Different people see different degrees of similarity, or recognize the
"importance" or value of such features that another may say are prominent
and figure in relationship. Then there's the operative separation between
states of a recognized character, and the perspective on absolute
condition versus gradation of the condition or between conditions. Two
dissimilar characters may actually be related as a functional complex, and
hence would be the "same" character, whereas others would separate this
and say they were two characters. Hence, there would be a great deal of
operationaol disagreement in what the value of similarity is. Many taxa
are unique based on absolutely definite apomorphies (loss of a finger,
etc.) that compare to other similar features, and this may be a
consideration of what to use as a meter, but otherwise, I personally
cannot see any value to a morphological meter, and Sibley and Alquist's
method is relative to which part of the genetic code you use. And with
both, there is a great deal of consideration neccessary for convergence or
mutation. Applying meter to bacteria and such simple non-Eukaryotic
organisms and simple Eukaryota is endangered by the nature of increased
drift, lateral gene transfer, and so forth. Bacteria and Archaea are much
more susceptible to genetic similarity whatever their evolutionary
descent, and this matters on applying such a meter to _them_. Some would
advocate a universal paradigm, then ... but I would say that such a
universal measure would be futile, and that a plastic, relative measure
may be required, as well as a concept of morphological versus molecular
when it comes to the absence of one or the other (for instance, a unique
artiodactyl from Vietnam was first identified genetically by its pelt, and
no osteology or myology was possible).

  Cheers,


=====
Jaime A. Headden

  Little steps are often the hardest to take.  We are too used to making leaps 
in the face of adversity, that a simple skip is so hard to do.  We should all 
learn to walk soft, walk small, see the world around us rather than zoom by it.

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