The Structure of Evolutionary Theory

(Michael S) #1
9 - 32. Cope's Law shown, under a speciational perspective, as a differential movement of
speciation events towards larger size from a constraining boundary imparted by a small founding
member of the lineage. Adapted from Stanley, 1973.

that the majority of generalized species also tend to be relatively small in body size
within their clades.
These statements still suggest nothing new so long as we continue to frame
Cope's Rule as anagenetic flux in an average value through time—that is, as a
conventional "trend" under lingering Platonic approaches to macroevolution. But
when we reformulate the problem in speciational terms—with the history of a
Cope's Law clade depicted as the distribution of all its species at all times, and with
novelty introduced by punctuational events of speciation rather than anagenetic
flux—then a strikingly different hypothesis leaps forth, for we now can recognize a
situation precisely analogous (at one fractal level down) to the previous
construction of life's entire history: an evolving population of species (treated as
stable individuals), in a system with a left wall of minimal size (for the given
Bauplan), and a tendency for founding members to originate near this left wall
(Fig. 9-32).
Therefore, just as for all of life in my previous example, if the clade prospers
with an increasing number of species, and even if new species show no directional
tendency for increasing size (with as many species arising smaller than, as larger
than, their ancestors), then the mean size among species in the

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