Paraphyly and names
tdib at OCEANCONSERVANCY.ORG
Fri Jan 18 15:01:52 CST 2002
From: SKÁLA Zdenek
It is a usual, but rather unfounded idea that apomorphies mean "having
something" and plesiomorphies "not having something", so the latter are
less informative and/or less real.
Here is a more accurate formulation.
Apomorphies are evolutionary novelties referenced to the historically
correct lineage branch (the one at which they actually arose).
Plesiomorphies are evolutionary novelties refernced to the wrong lineage
branch (one of the sub-branches beneath the correct branch).
Thus the amnion is an apomorphy of Amniota - discovery of the apomorphy is
evidence of the divergence of the lineage - i.e. it is good and useful data.
The amnion is a plesiomorphy of Homo sapiens- the character carries no
information regarding the divergence of Homo sapiens. At this level of
analysis, the character is irrelevant.
while there is a straightforward logic why synapomorphies are
crucial for constructing cladogram, there exists no logical reason why
they should be *exclusively* used for splitting the cladogram into taxa.
Oh yes there is. The reason is that synapomorphy (evolutionary novelties
recognized at their true level of generality) is the only character-type
that offers evidence for historical groups.
If, e.g., we have a pectinate cladogram of species A,B,C,D,E and clades
A+B and A+B+C+D are supported by many synapomorphies, we will gain the
most informative taxa by grouping ((A+B)+(C+D))+E.
That depends on what kind of information you wish to represent. If you wish
to deliver information about the history of the taxa, then no, your
formulation is not more informative. If you wish to make some general
subjective comment about the extent to which different groupings of species
might resemble each other, then sure, your arrangement might work.
Personally, I am committed to a systematics that seeks to represent the
history of life.
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