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Date: Mon, 25 Feb 2002 14:21:42 -0600
From: "Jonathan R. Wagner" <jonathan.r.wagner@mail.utexas.edu>
To: "T. Mike Keesey" <tmk@dinosauricon.com>
Cc: -PhyloCode Mailing List- <PhyloCode@ouvaxa.cats.ohiou.edu>
Subject: Re: interesting style of definition
TMK wrote (quoting me):
> > The only objective measure of relationship is what Darwin so succinctly
> > termed "propinquity of descent," frequently phrased as "recency of
common
> > ancestry."
>
> These seem like different terms to me, "propinquity" meaning "nearness".
Well, the important word being "descent." "Nearness of descent" and
"recency of common ancestry" address the same concept from two opposite
directions (up and down the lineages, if you will).
> -+-A
> `-+-B
> `-------------------------------------------------------C
>
> Some might say that B is nearer in descent to A than to C. Or maybe I'm
> misunderstanding the phrase. (I probably am.)
I think C is pretty unequivocally "nearer in descent" to B, in that
their lineages (the trail of ancestry and descent leading from each back
through time) share a longer common segment than either does with A. Or, to
put it another way, their lineages diverged more recently than their common
lineage did with C. Or an even better way: they share a community of descent
independent of C. They share community of descent independent of C, and in
turn this group will have a similar relationship with other taxa, and so on,
proceeding by incrementally quantized degrees of propinquity of descent.
The diagram above represents a nonunique solution: there is no objective
measure of anagenetic distance, at least for (phenotypic) morphology. There
may be for genetic distance, I just don't know. By my understanding, this is
why phenetics and gradistics both fail to provide an objective method of
classifying organisms... the "distance" factors are ultimately
unreproduceable (and there are philosophical reasons why this should be so).
Hence, propinquity of descent is the only (theoretically) objective means of
delineating named groups of species (or organisms, if you don't care for
that class of entity).
Hope this helps,
Wagner
Jonathan R. Wagner
9617 Great Hills Trail #1414
Austin, TX 78759