1034 THE STRUCTURE OF EVOLUTIONARY THEORY
have happened under our usual views. Thus, this second, conceptually positive
meaning of "constraint" embodies a relative concept that might be feared by those
who enjoy the comforts of power, but should provoke the delight of all scientists.
To cite two recent examples of this relative meaning from recent evolutionary
literature, Weiss (1990) wrote an iconoclastic paper arguing that geometric laws of
serial repetition, combined with limited structural paths of alteration, dominate
directions of phyletic change. (By the way, I disagree with his conclusion and am
only discussing his terminology.) In this context, the usual orthodoxy of ordinary
natural selection, working towards optimality in local adaptation, will be judged in an
opposite manner as an annoying trifle that might falsify the grand pattern and
temporarily hide its effects—in other words, as a constraint upon the regularity of
geometrically predictable transformation. Weiss (1990, p. 21) acknowledged that
natural selection occurs, but he dismissed the process as local distortion: "The
pervasiveness of metameric 'duplication with variation' shows that it is a central
principle of evolution... Despite... the pattern-distorting effects of selection and
drift, this evolutionary strategy is essentially unidirectional."
Weiss's taxonomy of concepts, so peculiar to those of us with Darwinian
training, makes sense in his system. We would never think uniting selection and drift
into the same category, for we view them as opposite processes with respect to our
primary interest in adaptation. But, within a theory of predictable linear change
enjoined by geometric principles, both drift and selection operate as local oddities
that distort a broader and fundamental pattern. In any case, when we note how
selection becomes a constraint upon a structuralist theory of geometrically rule-bound
transformation, we can understand more easily why internal channels of preferred
variation would be labeled as constraints upon a theory that ascribes all evolutionary
direction to natural selection. (When a rebel labels one's own central belief as a
limiting constraint, the generality of the usage becomes startlingly clear!)
In another example, Jackson and Cheetham (1999) cite punctuated equilibrium
as constraining because phylogenetic patterns generated by this theory preclude
several classes of results predicted by orthodox selectionist models of gradualistic
anagenesis in populations. They write (1999, p. 72): "The realities of punctuation and
stasis need to be better incorporated into evolutionary studies. Punctuated speciation
does not contradict conventional neodarwinian mechanisms, but it does constrain the
range of probable evolutionary scenarios for speciation, evolution of life histories and
macro-evolutionary trends." "Macroevolutionary trends," they add in explanation (p.
76), "must arise through differential rates of origination and extinction, and not by
adaptive evolution within single species."
Several participants in debates about the evolutionary meaning of constraint (see
Gould, 1989a) have explicitly embraced this relative definition. Stearns (1986), for
example, properly rejected a usage so overly broad that the term would then lose all
meaning—namely, the designation of all cause as "constraint" because any active
force must direct change in one way rather