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Re: Paraphyly and sister taxa



In a message dated 9/3/00 11:50:49 PM EST, kinman@hotmail.com writes:

<< The latter concept is only a Hennigian "convention", and in reality 
 almost all speciation events are actually paraphyletic mother species 
 budding to give rise to a daughter species.  The mother species survives the 
 speciation essentially unchanged.  Such Hennigian conventions are fine until 
 they begin to be taught as though they are real evolutionary processes. >>

Given two species A and B, there are six possible relationships they can have 
with each other: A is unrelated to B (that is, too distantly related within 
the context of the discussion); A is the direct descendant of B; B is the 
direct descendant of A; A is the immediate sister group of B with A as common 
ancestor; A is the immediate sister group of B with B as common ancestor; and 
A is the immediate sister group of B with a third species (neither A nor B) 
as common ancestor. But there is only one possible cladogram with A and B, 
namely, A and B are sister groups. So a cladogram is not a very good model 
for phylogeny.

There is only one sensible way to define a species within an evolutionary 
context, where we cannot observe the living organisms, and that is by the 
appearance of the concrete evolutionary novelty that distinguishes the 
species from its ancestor and the other species in its clade. That's where 
one draws the line between species in a lineage. No evolutionary novelty, no 
new species.