Punctuated Equilibrium and the Validation of Macroevolutionary Theory 935
daily to weekly between the ages of 3 days and 21 months, Lampl et al., using the
language and concepts of punctuated equilibrium, found that "90 to 95 percent of
normal development during infancy is growth-free and length accretion is a
distinctly saltatory process of incremental bursts punctuating background stasis"
(p. 801). In fact, Lampl et al. did not detect the pattern of quickness in change and
prevalence of stasis until they measured subjects at their finest daily scales (for
even the semi-weekly and weekly measurements smoothed out punctuations over
intervals of stasis). They conclude (Lampl et al., 1992, p. 802): "Human length
growth during the first two years occurs during short (less than 24 hours) intervals
that punctuate a background of stasis. Contrary to the previous assumption that the
absence of growth in developing organisms is necessarily pathological, we
postulate that stasis may be part of the normal temporal structure of growth and
development."
Lenski's bacterial populations generate large numbers of mutations (some 10^6
every day in each population, by the estimate of Elena et al., 1996). But the step-
dynamics revealed by the finer scheme of sampling—a pattern "predicted ... by a
simple model in which successive beneficial mutations sweep through an evolving
population by natural selection" (Elena et al., 1996)— presumably occur for two
reasons: first, the well-known exponential principle, however intuitively
paradoxical for most people untrained in science, that "many generations are
required for the beneficial allele to reach a frequency at which it has an appreciable
effect on mean fitness, but then relatively few generations are required for that
allele to become numerically dominant" (op. cit.); second, and probably more
important, the extreme rarity of favorable variants amidst the daily plethora of
mutations, leading to "a substantial waiting period before a beneficial mutation
even occurs" (op. cit.). Thus, at the proper scale (for resolving the causal
mechanism) of sampling every 100 generations, the plateaux of stasis mark the
waiting times between favorable sweeps, while punctuations express the sweep
itself.
Finally—and to place some substantial empirical weight upon the keystone of
my argument for the potential generality of punctuational change—Lenski and his
colleagues have greatly increased our understanding of evolution by developing an
artificial (in the best and fully positive sense of the word) experimental system
purposely reduced to an absolute "bare bones" of Darwinian causal minimalism.
With an identical genetic starting point for each replicate, an asexual clonal system
that permits no genetic exchange among cells, and an unchanged environment (the
regimen of daily transfer by controlled dilutions into a constant growth medium),
this experiment leaves only two factors free to work and vary as potential agents of
evolutionary change: the paired and essential Darwinian components of new muta-
tions as raw material, and shaping by natural selection among cells that vary as a
consequence of these mutations. The fact that punctuational dynamics prevail in
this first truly adequate experiment in pure Darwinian minimalism must at least
evoke a suspicion—even among biologists who, by custom and faute de mieux,
have never questioned gradualism—that this episodic mode might be expressing
something important about the general