934 THE STRUCTURE OF EVOLUTIONARY THEORY
it was necessary to make measurements at sufficiently high frequency to resolve
the punctuated dynamics."
This gain of insight by finer sampling raises the important methodological
theme that proper choice for a scale of inquiry depends crucially upon the
resolution needed to identify and characterize the underlying causal process of the
observed pattern—in particular, to specify the natural unit or entity of change in
the given system. (Such studies often face the paradox that, whereas the
recognition of this principle requires no act of genius, empirical adequacy often
founders upon a conceptual dilemma: We can specify a proper scale if we know
the causal basis beforehand. But, more often than not, we undertake such research
in order to discover an unresolved causal basis—thus bringing upon ourselves the
classical problem of a single equation with two unknowns: the causal basis and the
scale required for its identification, to complete the analogy.)
At the macroevolutionary scale of punctuated equilibrium sensu stricto,
events of speciation represent the natural unit, and geological resolution must be
sufficient to identify the occurrence and timing (relative to stasis, or any other
pattern, in the species's subsequent geological history) of origination for these
macroevolutionary "atoms." Several published studies have been fatally marred by
the basic flaw of using a scale so coarse that a trend generated by multiple events
of staircase speciation could not be distinguished in principle from the same result
achieved by smooth anagenesis in an unbranched lineage. In the most widely
discussed fallacy thus engendered, Cronin et al. (1980) claimed gradualism
(explicitly against punctuated equilibrium) for major trends in hominid evolution
because a temporal sequence of mean values moved in the same direction. But the
successive points were so widely separated in time and morphology that the
authors could not determine whether they had measured mean values of successive
species during their periods of stasis, or had sampled points in an anagenetic
continuum. (Punctuated equilibrium, after all, was proposed as an alternative
explanation for phyletic trends of this kind, not as a denial of their existence!) The
scale of measurement used by Cronin et al. may be compared with Lenski's first
procedure of sampling every 500 generations. Both schemes are too coarse to
"catch" the underlying causal unit of change—speciation, if punctuated equilibrium
holds, for the macroevolutionary case of hominids; the infrequent origin and sweep
of favorable mutations in bacterial anagenesis.
In a fascinating study, extending (to an utterly different realm of inquiry) the
generality of this important point about appropriate scale of measurement for the
recognition of punctuations, Lampl et al. (1992) note that human growth in body
length has generally, and for centuries of study, been regarded as smooth (albeit
highly variable in rate at different states of ontogeny) because "individuals have
been traditionally measured at quarterly intervals during infancy, and annually or
biannually during childhood and adolescence. Physiological data are
mathematically smoothed and growth is represented as a continuous curve" (1992,
p. 801). But by measuring a sample of 31 "clinically normal" Caucasian children at
intervals ranging from