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Re: "Cope's Rule" Put to the Test



Dinogeorge@aol.com wrote:
> 
> Name a genus of horse, fossil or recent, that is smaller than
> _Hyracotherium_. Name a tetrapod family or other clade most of whose included
> genera and species are of a >significantly< smaller adult size (factor of two
> or more) than its earliest-known member or hypothesized common ancestor. For
> every one you name of the latter, I'll name three that show the opposite
> trend.

There is something here that I'm not sure I understand.  As I understand
it, "Cope's Law" is simply the idea that in any tetrapod evolutionary
lineage, the individual species will tend to get larger over time?  Is
this correct?

If it is, then it appears to me that "Cope's Law" is an interesting but
ultimately unuseful generalized observation, for two reasons.  First,
most tetrapod clades start out small and get bigger because in general,
only smaller tetrapod animals survive mass extinctions and re-radiate
into new lineages.  Second, if evolution is an ultimately unguided
process, then each lineage will have its own selective reason for
getting bigger (or smaller) over evolutionary time, and there's no valid
grounds for extrapolating trends across lineages.  

Maybe I'm missing something (always very possible), but I keep getting
this image of somebody somewhere looking at a series of theropod fossils
and saying, "why did these theropods get bigger over time?  Must be
Cope's rule at work." and leaving it at that.  Uh-uh.  There has to be a
specific selective reason why that theropod got bigger.  Maybe it
switched primary prey, or maybe its prey got bigger, or maybe something
else.  But IMVHO, "Cope's rule" can't be invoked as an explanation for
anything, nor as a basis for conjecturing evolutionary processes.  At
worst it's an illusion; at best it's like saying "every liquid has a
freezing point" -- definitely true, but it doesn't actually contain much
useful information.  

-- JSW