Re: synapomorphies not created equal

[Dinogeorge@aol.com]

In a message dated 9/1/01 0:16:40 AM EST, kinman@hotmail.com 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 
 falls. >>

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? 
Why?

(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.