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



In a message dated 97-01-23 03:02:20 EST, ornstn@inforamp.net (Ronald
Orenstein) writes:

<< Even this formulation is not entirely true, as the fossil pygmy elephants
 from the California Channel and Mediterranean islands show (and also,
 probably, the Inaccessible Island Rail, just to give a maniraptorian
example).>>

The statement as I phrased it is quite correct, because it does not >insist<
that Cope's Rule be an exception-free absolute, merely a >tendency< that
admits exceptions such as those you mention. The scarcity of such exceptions,
among vertebrates at least (the only group with which I can claim to have any
expertise or experience), testifies to the one-way nature of this tendency.
 
<< What might be better to say is that as the vast majority of living animals
 are small, and as we all evolved from tiny critters somewhere down the line
 (Pikaia?), any animal above a certain size must,somewhere in its lineage,
 have evolved from smaller ancestors, and the ancestry of any really huge
 animal must include a number of creatures of gradually increasing size that
 could be placed in a temporal sequence (though there might have been some
 reversals along the way - at least some of the modern mammalian megafauna
 may (I think?) have recent ancestors that were larger than their modern
 descendants).  This situation might give the APPEARANCE of a general trend
 such as that stated in Cope's "rule", when it is really only an observation
 that really big animals must at some level have evolved gradually from
 smaller ones. >>

No, this is not what I'm saying at all. It is quite obvious and trivial that
all large animals must have evolved from smaller ones. But it is not at all
obvious that it is very difficult for the reverse to occur, namely, for small
animals to evolve from larger ones. I'm not talking about >trends< here, or
the appearance of trends by selecting certain lineages and ignoring others,
or any such stuff. What I'm saying is that a population of small animals will
evolve from significantly larger animals only under some pretty exceptional
circumstances, whereas such exceptional circumstances are not required for
the evolution of larger animals from smaller ones.

For example, the evolution of the horse. It was once used as an excellent
example of Cope's Rule, because horses started out as relatively small,
cat-size animals (_Hyracotherium_ and so forth) and ended up as relatively
large animals (_Equus_ and so forth). Then the anti-Cope's Rule crowd pointed
out that the equid family tree was much more diverse, that it's not correct
to single out one lineage, etc., etc. Well, the fact remains that NOT ONE of
the known descendants of _Hyracotherium_ (or whatever the common ancestor
was) was significantly >smaller< than _Hyracotherium_ (say, rat-size),
whereas >almost all< of them became significantly >larger< (sheep-size to
horse-size). So the equid family tree quite nicely exemplifies Cope's Rule
>as I have phrased it.< Maybe I should call it Cope's Tendency or something;
the word "rule" carries too much emotional baggage.