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Re: No Hedging.



>does this mean something like a difference as small as modern domestic
>animal variation due to selection?  Like trying to tell the difference
>between a Devonian Rex (curly-ratted coated cat) and a Scottish Fold
>(curled-over-ear cat) merely by looking at the skeleton (or a Golden Lab
>and a Black lab)? So VISUALLY, in person, the mice-and-elephant
>creatures could have been distinct from each other but internally they'd
>be nearly identical?
>
>Or would they have been even MORE alike than even that and just carried
>the new-and-separate genes that would allow for later spciation?

Most likely, the immediate descendents of the last common ancestor of
rodents, elephants, carnivorans, etc. were ecologically very similar to
that common ancestor, and the features distinguishing them had nothing to
do with those characters currently diagnosing Rodentia, Proboscidea, and
Carnivora.

In fact, the differences in the first generation immediately following
speciation are generally very subtle and expressed primarily at the
chromosomal rather than phenotypic level.

The actual degree of divergence - morphological or molecular - is difficult
to predict, as it will depend on many factors - generation time (in some
groups), reproductive mode (in some groups), the availability of ecological
vacuum, the relative strength of sexual over other kinds of selection,
population sizes (which might make drift an important factor), possible
morphological channelization (in some groups), and whatever other pressures
we haven't discovered yet.  It will also vary over time, such that those
animals living a million years after speciation will look more similar to
each other (on average) than those living ten million years later, and
rates of evolution (molecular or morphological) need not be the same in
different lineages, which is why many now view molecular clocks as things
to be demonstrated rather than presumed.






>
>-Betty Cunningham
>
>chris brochu wrote:
>>
>> >I respect your opinion--that genetic change doesn't
>> >necessarily create morphological change.  But it is just an hypothesis.
>>
>> Actually, this isn't entirely my opinion.  I think what people are
>> confusing is divergence date and divergence rate.  The lineages today
>> including elephants and rodents may have diverged in the Turonian, and they
>> may even have been morphologically distinct early in their separate
>> histories (though not necessarily recognizably rodent or elephantine).  The
>> "burst of radiation" in mammalian diversity during the Paleocene might have
>> involved an increase in diversification, but the basic lineages were
>> already in place.  Moreover, many of the lineages present in the Paleocene
>> are extinct and thus not available for molecular sampling.


-=--=--=--=--=--=--=--=--=--=
Christopher Brochu, Ph.D.

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