Punctuated Equilibrium and the Validation of Macroevolutionary Theory 877
punctuational breakdown in the history of human ideas and social organization (see
pp. 952-967).
CAUSALITY. Fruitful debate about the causes of stasis must first specify the level
manifesting the common phenomenology. (Obviously, causes of learning plateaus
in piano playing cannot be strict homologs of ecological reasons for joint stability
of species in coordinated stasis, even though the graphed pattern of change may
manifest the same geometrical form.) In this section, I confine my discussion to
punctuated equilibrium sensu stricto, and not to the general pattern of
punctuational change at any level—that is, to proposed reasons for the observed
high relative frequency of stasis during the full geological range of metazoan
species as preserved in the fossil record.
But first, and as an example of how discussion can proceed at cross purposes
when proper scales have not been specified, I must note that most of the literature
proclaiming punctuated equilibrium as "old hat" (Lewin, 1986) or something long
known and merely hyped by ill-informed paleontologists, has only analyzed
ecologically rapid anagenesis in populations rather than the relevant phenomenon
of cladogenesis by speciation scaled against subsequent geological duration in the
stasis of species so generated. Most notably (in terms of subsequent commentary),
two papers of the mid-80's (Newman, Cohen and Kipnis, 1985; and Lande, 1986)
developed mathematical models to show that single populations could move
rapidly (in the "ecological time" of a human career) from one adaptive peak to
another in the absence of environmental change. (The major previous stumbling
block had been set by problems in envisaging how a population could move down
an adaptive peak, against any force of selection, to inhabit a valley, and therefore
become subject to selection up an adjacent peak. The basic solution—that the
descent must be rapid—allows sufficient impetus against selective forces, and also
links the models to themes of speedy anagenesis.)
Lewin (1986) used these studies to write a news and views feature for Science
entitled "Punctuated equilibrium is now old hat," while also recognizing that
ecologically rapid anagenesis does not address the scale or level (not to mention
the reality of changing environments in our actual world) of punctuated
equilibrium's central concern. We welcome such plausible models of ecologically
punctuated anagenesis as a contribution to understanding the panoply of causes
that yield punctuational change at other levels. But this smaller-scale phenomenon,
however fascinating and important, bears little relevance to the causes of stasis
within species during geological time (or to the cladogenetic sources of geological
punctuation as a slow branching event in ecological time).
We may order the major propositions for explaining stasis at the scale of
punctuated equilibrium as an array running from conventional resolutions based on
Darwinian organismic selection to more iconoclastic proposals invoking either
higher levels of causation or less control by selection and adaptation. (Much of the
genuine interest in the otherwise tedious and tendentious