[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: copes rule



At 09:24 PM 1/27/97 -0500, you wrote:
>I got in on discussion late.
>
>so what is cope's rule?  
>

Something which is very much misunderstood by many people on the net... :-)

Cope (Edward Drinker Cope, that is, famous 19th Century vertebrate
paleontologist AND ichthyologist AND herpetologist AND general marine
biologist [as many of us within Vert Paleo sometimes forget]) actually
proposed several different "rules" which he saw governing the evolution of
life.  Unlike his contemporary and rival, O.C. Marsh, Cope was not a
Darwinist: he did not consider natural selection necessary nor sufficient to
explain evolution.  Like others of his time (and his student, H.F. Osborn),
Cope believed in some inborn forces which drove evolution in predetermined
directions.

The most famous formulation Cope offered, which is often called "Cope's
Rule", is "In animal taxa [note: NOT just tetrapod, or even vertebrate; but
animals], there is a long-term phylogenetic trend toward increased body
size".  To Cope, this was due to some inborn drive.  As others have shown
(for a classic: Steve Stanley, 1973, _Evolution_ 27:1-26), there are
mechanistic reasons why this might be true.  Specifically, for certain
fundamental body plans, a major decrease from the starting size may be
prohibited by physiology, biomechanics, reporduction, or other biotic factor
(can a tetrapod grow smaller than 1 g?), whereas the other direction
(increased size) may be comparatively open ended.

(The converese applies in some groups, viz. Arthropoda.  There are
biomechanical and physiological factors which prohibit one ton arthropods
and the like, but the small end of the spectrum is much more open.  Hence,
there are very few large arthropods, but godzillions of species of mites,
maxillopods, etc. which are smaller than your typical amoeba!).

Less famous than the trend labelled "Cope's Rule" is one of Cope's other
rules: the rule of the unspecialized (or the generalized: must check to see
how he named it).  Essentially, he stated that the forms which tended to
give rise to the most others forms are the unspecialized members of a taxon,
whereas the more specialized branches of the lineage tended to die off
without issue.

However, Cope did not offer quantitative surveys to support these ideas.
Much recent work (Stanley, mentioned above; MacFadden, on horses;
Jablonski's paper in Nature, which started this talk, and refs. in these and
in Gould's _Full House_) suggests that much of the "reality" of Cope's rule,
even as a general trend, is due to ideological blinders.  We tend to be
uninterested as professionals or amateurs in vertebrates (especially) in
cases which run counter to Cope's Rule (dwarf mammoths being a rare
exception).  However, when you sit down and go through a lineage with good
phylogenetic resolution and a good fossil record, you can find that other
patterns are present.

MacFadden's work on horses, for example, shows that phyletic size increase
from species to species is not statistically more common than phyletic size
decrease from species to species.  We happen to be living in a depauperate
fauna of equids (only Equus, the largest genus, survived into the Recent).
If this were a million years ago or so, Nanippus (which was gazelle sized or
smaller, and thus comparable in size to some of the Oligocene horses) would
still be around, and horses would probably not be used as the examplar of
phyletic size increase.  (As Gould and others point out, we have a diverse
antelope fauna still alive, with a very good fossil record.  The fact that
there ISN'T a major phyletic trend in size or other simple feature means
that the bovids don't tell a simple story, so the textbooks don't deal with
it, so most people remain ignorant of this well-studied evolutionary lineage).

If I remember, I will try to bring in the refs for all these papers
tomorrow, including Cope's orignal work.

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:th81@umail.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661

"To trace that life in its manifold changes through past ages to the present
is a ... difficult task, but one from which modern science does not shrink.
In this wide field, every earnest effort will meet with some degree of
success; every year will add new and important facts; and every generation
will bring to light some law, in accordance with which ancient life has been
changed into life as we see it around us to-day."
        --O.C. Marsh, Vice Presidential Address, AAAS, August 30, 1877