Re: Apomorphy-based definitions

[Dinogeorge@aol.com]

In a message dated 1/27/02 1:59:06 AM EST, qilongia@yahoo.com writes:

<< Drop apomorphy-based definitions... >>

Considering that we observe the appearance of a new species in the fossil 
record by the establishment of an evolutionary novelty, and not by which side 
of a cladogram node a fossil lies on, I think it may be quite premature to 
drop apomorphy-based definitions for clades. Say a species splits into two 
isolated populations, A and B. After a while, A and B accumulate characters 
that distinguish them (and prevent interbreeding) at the species level. Where 
do we draw the line and say that populations A and B have become different 
species? At the split (which is where a cladist might draw the line), when 
they were still the >same< species, or afterwards, when they have acquired 
evolutionary novelties and >really are< different species? Given a fossil, 
how do you tell which taxon it belongs to? By the characters it exhibits--not 
by trying to guess whether it falls above or below the split. Apomorphy-based 
definitions are truer than node-based definitions.

For every clade, there is exactly one character that distinguishes it from 
its ancestors. I call this character the key character or key apomorphy for 
the clade, and the appearance of the key character defines the observable 
origin of the clade. Not the node, the key character. Given the continuous 
nature of the tree of life, it is extremely unlikely that two or more 
characters will appear in a lineage at >exactly< the same time, so as to 
present two or more potential key characters simultaneously. One or the other 
will appear first, and it is up to the investigator to pick the one that is 
the key character and thereby to anchor his taxon.

In subsequent evolution of the clade, the key character may be lost. That's 
not important; it is the presence of the key character at the time the clade 
came into existence that is important. Loss of a character can define a 
subclade just as the gain of a character can define a clade.