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Darwin-L Message Log 3:87 (November 1993)

Academic Discussion on the History and Theory of the Historical Sciences

This is one message from the Archives of Darwin-L (1993–1997), a professional discussion group on the history and theory of the historical sciences.

Note: Additional publications on evolution and the historical sciences by the Darwin-L list owner are available on SSRN.


<3:87>From mayerg@cs.uwp.edu  Mon Nov 22 10:12:26 1993

Date: Mon, 22 Nov 1993 09:42:51 -0600 (CST)
From: Gregory Mayer <mayerg@cs.uwp.edu>
Subject: Re: phenetics vs cladistics vs evol. class.
To: darwin-l@ukanaix.cc.ukans.edu

> 1) My understanding was that Mayr said there is no method in Cladistics for
> going from phylogenetic reconstruction to classification.  Comments?
	Sort of true.  Cladistic classifications reflect only recency of
common ancestry (as opposed to degree of divergence).  This imposes a
strong constraint on such classifications, if they are to be of the
traditional Linnaean style.  The constraint is that sister groups (i.e.
the pair of clades descending from a single common ancestor) must be of
the same rank.  Thus for a particular phylogeny, many possible
classifications will not satisfy this constraint, and thus will not be
cladistic.  However, many possible classifications _will_ satisfy this
constraint; there is thus no procedure that moves one ineluctably from a
tree to some single classification, which alone is cladistic.  The
cladist, while being constrained more than a pheneticist or evolutionary
classificationist (sorry for the barbarism, but evolutionist without some
reference to classification would not be right) would be, must still make
decisions as to which rank sister taxa should be, and which sister taxa
should be named (in a practical system, not all sister taxa can be named).
This is what Mayr meant by saying cladistics had no method.

> 2) My understanding is that a major purpose of cladistics is to determine
> -branch points.-  What EXACTLY is a branch point?  Is cladistics concerned
> with determining branch points?
	Evolutionary events are often divided into two sorts: anagenesis,
or changes within a single lineage, and cladogenesis, or the splitting of
a single lineage into two or more lineages.  Here's an example, which
might, in fact, be wrong, but it illustrates the point.  Ribbon snakes
occurred across the eastern part of the U.S.  During one of the climatic
vicissitudes of the Pleistocene, acceptable habitat became restricted to
the east (toward Florida) and the west (toward Texas), separating the
ribbon snakes into two geographic areas.  In the separate eastern and
western areas, evolution continued , but independently.  The snakes
changed a bit in their coloration and scales and probably other ways.
Thus anagenesis occurred in the isolated populations.  When the climate
ameliorated, snakes moved back into once-again favorable areas along the
Mississippi.  When eastern snakes and western snakes met, however, they
could no longer freely reproduce with one another.  They had become
separate species, i.e. independently evolving lineages.  Cladogenesis had
occurred.  The complex of events leading to the splitting of the ribbon
snake into two species, the eastern and western ribbon snakes, is what is
referred to as the branching point: the point (although, of course, in the
postulated example it was not a _point_ in time, but a sequence of events)
at which a single branch of the phylogenetic tree becomes two branches.
The goal of cladistics (as a branch of phylogenetics, as opposed to
classification) is to discover the sequence in which these branching
events occurred.

Gregory C. Mayer
mayerg@cs.uwp.edu

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