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Re: furball size



In a message dated 98-05-07 12:54:16 EDT, tonyc@foe.co.uk writes:

<< Most of your points I'd agree with, but not the last one [which is why
 Cope's 'Rule' has fallen out of favour].  There are many examples of
 reduction in size in lineages.  S J Gould has pointed out that the
 differential extinction of larger forms in mass extinction events leaves
 much more scope for increase in body size than decrease, giving the
 illusion of Cope's Rule. >>

If we stop calling it Cope's Rule and call it Cope's Tendency (or something
like that), we'll have less trouble with it. Among terrestrial vertebrates,
lineages that hold to Cope's Tendency seem to outnumber lineages that do the
opposite by about three to one. That's certainly a good tendency, definitely
not a rule. Gould has provided a partial >explanation< for the tendency, but
he has certainly not shown that it does not exist or that it is an "illusion."
Actually, I suspect that the real tendency is for species to remain about the
same size, becoming neither smaller nor larger. But when there is a size
change, the tendency is to become larger.

<<I would point out, without further comment, that both extant endotherms
(bats, shrews, hummingbirds) and ectotherms (many fish, frogs, geckos)
have lineages with extreme size reduction.>>

Each of these lineages requires only a single size reduction in an ancestral
form, after which the descendants remain approximately the same size. Most of
the animals in each of those groups are of about the same size.