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Cope's Rule



Hello--

        I was interested to see Cope's Rule brought up on the list, as
that's a pet interest of mine. The conclusion I came to after writing a
paper for David Wake's Evolution course was that it was pretty useless
trying to explain Cope's Rule, whether you approach it from a
"macroevolutionary" (e.g. Gould's "trends as increases in variance"
viewpoint) or a "microevolutionary" (e.g. putative inherent advantages of
larger size a la Depperet, Simpson, etc.) viewpoint. Cope's Rule is simply
a generalization (mean body size of a lineage generally tends to increase
over time) that may or may not hold, depending on the lineage in question
and the time scale considered. It's really not terribly interesting.

        What interests me and many evolutionary biologists are the specific
cases within specified clades -- Jablonski's mollusks, Osborn's
proboscideans, Lister's red deer, Roth's dwarf mammoths, and, to cite a
dinosaurian example, our beloved tyrannosaurids. The patterns of body size
change are of some (perhaps trivial) interest, but the processes underlying
these patterns are the Holy Grail of "macroevolution". Unfortunately, these
processes are basically unknowable to paleontologists, so perhaps we should
look to recent studies of "microevolution"? (Man, do I ever hate that
dichotomy)

        If we do look to studies of body size changes in recent animals --
tiger snakes on the Chappel (sp?) islands, for example -- we find that the
processes acting are often too multifarous to point out one overarching
principle (e.g. safety from predation, or ability to procure and eat large
prey) as the causative process. Perhaps then, do we take another sort of
hierarchical approach and look at the underlying genetic and epigenetic
developmental processes behind body size change? Again we are stymied. What
it comes down to is a quantitative genetics approach: "body size" is a
mozaic of correlated traits, so it is inevitably a compromise between many
factors that are intrinsic or extrinsic to the organism, and we can attempt
to describe it as an "infinite-dimensional matrix". What does this mean?
Well, it's pretty obvious if you consider what body size really is. It's
not just a number, or a data point on a plot. It's the volume of the
olecranon process, plus the diameter of the orbit, plus the surface area of
the articular facet of the distal fourth metatarsal, plus... well, you get
the point I think. How do we ultimately explain evolutonary changes in body
size, then?

        In most cases, maybe we don't. But there are "natural experiments"
that may offer some clues -- Peter Grant's work with Darwin's finches being
but one of many. What these case studies have shown is that (surprise!)
body size evolution is case-specific, and is best (or perhaps only)
explained on a case-by-case basis, with processes that are deducible from
the visible natural patterns. Please, _please_ note that I am keeping
_explanation_ vs. _description_ of patterns separate. An "increase in
variance" does not explain body size change; it describes it.

        Cope's rule? Law? It's just better to get away from chasing
Victorian fantasies of formulating inviolate, omnipotent evolutionary
"laws" and settle with what we can explain from the patterns visible in
nature. When it comes to dinosaurs or other extinct lineages that show
interesting patterns of body size changes, the profound depth of time in
question places these processes basically out of reach. We may be able to
formulate explanatory hypotheses that are putatively falsifiable, but never
actually test them, or rule out alternative or corollary explanations.

        I hate to be a wet blanket, but maybe we're left with description
of evolutionary body size changes as the only option. Instead of asking
_why_ body size changes, studies with long-extinct lineages offer potential
of showing _how_ body size changed, and what changed with it. I'd sure like
to see (and I plan to do if no one else does) changes in dinosaur body size
plotted onto a phylogeny, although there might be some niggling problems
with circularity. There are plenty of interesting questions there, like
changes in locomotory abilities (my personal holy grail) or
ecomorphological questions a la Van Valkenburgh.

OK, enough ranting and pontificating. That's my opinion, love it or leave
it. Bye!




                        John R. Hutchinson
                 Department of Integrative Biology
                  3060 Valley Life Sciences Bldg.
                University of California - Berkeley
                        Berkeley, CA 94720
                      Phone:  (510) 643-2109
                      Fax:    (510) 642-1822
         http://ucmp1.berkeley.edu/people/jrh/homepage.html