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Re: splitting headache



 From: "Considine, Blaise" <bpc.apa@email.apa.org>
 > 
 > No, this isn't a flame. With the recent talk of splitting, especially 
 > regarding stegosaurus in the Morrison formation, I was wondering, just 
 > when would a species become a species?

The biological reality that one is *trying* to estimate is *effective*
reproductive isolation.  (Note, the occasional hybrid, especially in
disturbed habitats or zoos does not indicate lack of isolation].

In extinct fossil groups it can be very difficult to get a handle
on this issue.  Only where large sample are available, allowing
population statistics to be derived, is it possible to get direct
evidence for reproductive isolation or lack thereof.  Otherwise
one must make educated guesses based on the knowledge about normal
intra-specific variation in ecologically comparable forms today.

 > That is, what differentiates two 
 > species from each other rather than them being cases of a) individual 
 > differences within a species occurring at the same time, or b) changes 
 > within a species that have occurred over a period of time.

Actually, if the latter reaches a level where it would produce effective
reproductive isolation from the prior form, then it is still a valid
new species.

 > If I've gotten 
 > anything from the cladistics discussions, would the difference lie in the 
 > occurrence of a particular characteristic?

Cladism does not really deal with the species level.  It deals with
populations and traits.  It can be the case that a single biological
species spans several nodes in a cladogram.  I know of an actual
case involving two living species of insect (beetles I think), where
the more widespread species has several geographic variants.  The
cladogram of these morphs makes the distinct species a sister group
of one of the "races" of the cosmopolitan species, rather than the
sister group of the whole species.

swf@elsegundoca.attgis.com              sarima@netcom.com

The peace of God be with you.