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Re: synapomorphies not created equal
In a message dated 9/1/01 0:16:40 AM EST, firstname.lastname@example.org writes:
<< I've been thinking very hard about this, and I am still not convinced
that all synapomorphies are created equal. Some are "stronger" than others
no matter how long or big the evolutionary gap happens to be in which it
Indeed, we can be certain that all synapomorphies are >not< equal. And we can
likewise be certain that there is simply no way to decide which
synapomorphies are stronger than others--not even after an analysis is
finished. This is one of the central problems of cladistics: What weight
should be assigned to a particular feature in doing cladistic analysis?
Current cladistic analysis is almost always done unweighted, so that every
character has the same weight as every other character. But there is no way
to decide whether this particular weighting system is better than any other
weighting system. How can we say whether a feature such as "meat chopper
haemal arches" is worth the same as "pelvis propubic" or "postfrontal
absent"? What would "worth" even mean in this context, and how would you
measure it? Is a feature worth less just because it appears in a single
small, less inclusive group rather than in one large, more inclusive group?
(I've posted on this subject a number of times before.)
Incidentally, any clade is united not by a list of synapomorphies but only by
a single synapomorphy; in any list of synapomorphies, some must inevitably be
synapomorphies of nested groups contained in the clade, and others must be
plesiomorphies of clades that contain it. This is because it is extremely
unlikely that a set of apomorphies would all evolve at exactly the same time.
Apomorphies appear in a lineage in serial order, sometimes in rapid
succession, sometimes more slowly. When they appear in rapid succession and
the fossil record is poor (as usual), it presents the illusion that they
appear all at once.
For example, two apomorphies that are said to unite Ornithischia are the
presence of a predentary bone and the presence of an opisthopubic pelvis. One
of these must have appeared first, but the fossil record is too poor to tell
which. Suppose the predentary appeared first. Then it's not a synapomorphy of
Ornithischia; it's a plesiomorphy, because there was at least one
non-ornithischian animal that had a predentary but not the opisthopubic
pelvis. (Likewise if the opisthopubic pelvis appeared first.) By definition,
the only way a feature can be a synapomorphy of a clade is if it appears in
organisms within the clade but not outside the clade.