874 THE STRUCTURE OF EVOLUTIONARY THEORY
(1) if an established or an incipient species experiences a widely fluctuating
environment on geological timescales, the evolutionary response
(morphological change) tends to become damped with time, and (2) those
species that are least sensitive to environmental change (the most
"generalized" in a long-term sense) are the ones that tend to persist,
remaining in morphological stasis until a threshold is reached (Sheldon,
1996, p. 218).In a second extension, Sheldon makes an almost quizzical, but oddly
compelling, argument based on another important source of potential correlations,
previously unaddressed here but perhaps quite important: time itself, expressed
either as the absolute time of particular intervals in the earth's history, or as the
relative time of distinctive segments in the general "ontogeny" of a species's
duration. Many biologists have noted the apparent paradox that so little sustained
and directional evolution (as opposed to abundant evidence for rapid and adaptive
fluctuations in such characters as bill form in Darwin's finches or wing colors in
peppered moths) has been noted for species in historic, and recent prehistoric,
times during the tenure of modern humans (who have also remained in stasis) on
earth. I would, of course, attribute this phenomenon mostly to a general prediction
for stasis in the vast majority of lineages at any time (while charging our
puzzlement only to the false equation of evolution with gradual change). But
Sheldon raises the interesting ancillary argument that this general expectation may
now be enhanced by special advantages for stasis in the regimes of strong and
rapid worldwide climatic fluctuation that our earth has been experiencing in these
geologically unusual times: "Given the Quaternary climate upheavals, relatively
little evolution may be occurring worldwide at present (except for evolution
induced by humans)" (Sheldon, 1996, p. 209). I can only hope that the more
punctuated equilibrium induces change in our evolutionary views, the more things
will not be the same in our interpretations of the history of life.
The Broader Implications of Punctuated Equilibrium for
Evolutionary Theory and General Notions of Change
WHAT CHANGES MAY PUNCTUATED EQUILIBRIUM INSTIGATE
IN OUR VIEWS ABOUT EVOLUTIONARY MECHANISMS AND
THE HISTORY OF LIFE?The explanation and broader meaning of stasis
As emphasized throughout this chapter, the stress placed by punctuated
equilibrium upon the phenomenon of stasis may emerge as the theory's most
important contribution to evolutionary science. The material world does not impact
our senses as naturally and objectively parsed categories. We can make accurate
observations and measures of particular "things," but the ordering of "things" into
categories must be construed largely as a mental operation based on our theories
and attitudes towards "reality." Moreover, we