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Re: [Re: Reptilia]

In a message dated 12/8/98 5:24:49 PM Eastern Standard Time,
tkeese1@gl.umbc.edu writes:

<< The only problem I have with the cladistic definition is that it does not
 > one what the diagnostic features are.  All it says is the most recent
 > ancestor.
 The definition is not concerned with features. That's up to the diagnosis. >>

> So what constitutes being the common ancestor?

Being the most recent critter which gave rise to all of the anchor taxa,
natch. How can you tell if a particular organism is the MRCA? You can't.

I'm trying to follow this, and I don't think I've simplified the argument.

>From the statements above:
Descendants of a common ancestor are diagnosed by certain characteristics.  [A
newly discovered animal would or would not be considered a reptile based on
whether it has these characteristics.]  The common ancestor is not necessarily
a specific, identified animal.

Therefore, the common ancestor must also be diagnosed by, even deduced from,
the same characteristics.  If the common ancestor is unknown, it cannot be the
starting place for identifying a grouping of animals.

>From this logic, the characteristics, not the ancestor, act as the definition,
the common elements uniting all the members of the set.  
However, the intention of this definition is to avoid having the
characteristics define the set of animals.
Therefore, an additional assumption exists and must be directly associated
with all of the characteristics.  The only type of assumption I can think of
that would unite a set of physical characteristics is that they are for a
single purpose, or related set of purposes.  In other words, an adaptation.

The definition based on common ancestry therefore implicitly identifies a set
of animals which meet the selected set of problems a,b,c... by ...x,y,z.  The
characteristics are (at least the physical aspect of) x, y, and z, and the
definition assumes that we know what a,b,c are.  (In my case, this may be
excessive confidence.)  The presence of a characteristic implies its purpose
and the purpose is the core of the definition.
The purposes selected apparently control classification.

I suspect that I'm missing something, logical, but missing something.