"polarized"/holophyletic (precision)

Thomas DiBenedetto tdib at OCEANCONSERVANCY.ORG
Mon Jan 7 13:47:15 CST 2002

-----Original Message-----
From: Ken Kinman

Dear All,
     I stand corrected on the biological definition of "polarization".  The
"Mr. Spock" in me still thinks it is illogical that the term "ordering" was
restricted only to cases where you have 3 or more character states.  But
usage is what counts, so I will not belabor it further.            :-)
          ------ Ken

Dear Mr. Spock,
Ordering establishes the adjacency of character states, which can be linear
(e.g. 1,2,3 or 1,3,2) or not (think of the three states at the angles of a
triangle). With two states, there are no ordering decisions to be made,
since the 2 states are necessarily adjacent. So, if you prefer, you can
consider that ordering is not restricted to cases of 3 or more states, but
is simply a moot issue with 2 states. Polarization is, as has been pointed
out, the proposal of evolutionary directionality to the states. This is
often done "en masse" for all characters through the designation of an
outgroup (i.e. through rooting the tree).

P.S.  However, if we are going to be precise, it is even more important that
we should oppose the usage of the term "monophyly" as it has been
cladistically redefined.  That was so illogical, that Ashlock had to propose
the term "holophyly" (in 1971) to remedy the situation

The term was not really redefined, rather the definition was simply refined.
>From "a grouping with an exlusive common ancestor" to "a complete grouping
with exclusive common ancestor".  This seems to me to be very much in the
spirit of the original definition - monophyly was meant to refer to the
natural groupings that evolutionary diversification had produced, in
distinction to non-phylogenetic groupings (groups with no common ancestor
exclusive to themselves - polyphyletic). With the renewed dedication to
complete and accurate representation of the tree of life, there was a need
for a more precise formulation of monophyly, to separate out groups that
might pass the original criterion of having an exclusive common ancestor,
but which were incomplete. Paraphyly is a very good word for this, and one
that Ashlock accepts. The cladistic proposal made perfect sense, which is
why, I imagine, it has become standard usage. It is Ashlock's proposal that
strikes me as illogical because it proposes unrefining the definition of
monophyly and using it as a general term to encompass both paraphyly and
"holophyly". Of what use is such a general term?  The work of systematics is
focused primarily on distinguishing between the two types of groups, rather
than ever making statements about both categories.

Tom DiBenedetto

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