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Re: Paradoxically temporal



>In a message dated 7/23/98 8:34:00 AM Eastern Daylight Time,
>Steve.Tomporowski@us.ms.philips.com writes:
>
><< The phylogenetic species concept would argue that the ancestral species
>goes extinct upon speciation, even if one of the daughter lineages is
>indistinguishable from the ancestral lineage. >>
>
>If a lineage is *indistinguishable* from its ancestral lineage, on what basis
>is it identified as a *daughter* lineage?

(I think I'm the one who originally wrote the stuff in quotes, so I'll
respond to it.)

It may not be distinguishable.  This is one reason why some workers -
myself included - would argue that direct ancestors can only rarely be
positively identified.

The only positive evidence available that one species actually produced the
ancestral population for another is paraphyly of that species - something
that is very hard to demonstrate.  In nearly all cases, statements that
"taxon A evolved into taxon B" are falsifiable hypotheses, not conclusions.
If taxon A is older than taxon B, shares synapomorphies uniquely with taxon
B, and displays no autapomorphies of its own, then our hypothesis remains
unfalsified.

(Again, I'm restricting my discussion to cladogenesis.  There have been
several studies showing anagenetic change in both living and extinct taxa.
This is a different phenomenon.)

That such hypotheses can be very weak should be self-evident.  Lack of
autapomorphies could result from nonpreservation - rip the feathers from
ten different species of North American warbler, and I defy you to
recognize more than one.  And for some groups, stratigraphic separation can
mean very little.

This brings up another issue - the inclusiveness of a species-level taxon.
Any extinct species will be more inclusive than a living one.  Consider the
feather example above - if warblers were an extinct lineage, we wouldn't
come close to diagnosing the number of species currently known.  Current
molecular work is splitting living species further, so that even a field
guide won't help you - you'll need to know the calls and geographic ranges.
It's entirely possible that the taxon currently known as Maiasaura
peeblesorum is actually five different species, separated only by the color
of the iris or honking note - we'll never know.


chris




-=--=--=--=--=--=--=--=--=--=
Christopher Brochu

Postdoctoral Research Scientist
Department of Geology
Field Museum of Natural History
Lake Shore Drive at Roosevelt Road
Chicago, IL  60605  USA

phone:  312-922-9410, ext. 469
fax:  312-922-9566

cbrochu@fmppr.fmnh.org