Punctuated Equilibrium and the Validation of Macroevolutionary Theory 933
Punctuational patterns occur at the two different scales of overall trajectory
and detailed dynamics, even within the limited scope of this study. In the 1994
paper, Lenski and Travisano sampled each of the 12 populations once every 500
generations. They noted a rapid increase in mean cell size, well fit by a hyperbolic
model (see Fig. 9-34), for the first 2000 generations in each population, followed
by several thousand subsequent generations of little or no further increase—a
pattern that they described as punctuational in one of the major conclusions of their
paper (1994, p. 6809): "For ca. 2000 generations after its introduction into the
experimental environment, cell size increased quite rapidly. But after the
environment was unchanged for several thousand generations more, any further
evolution in cell size was imperceptible ... These data therefore indicate a rapid
bout of morphological evolution after the population was placed in the
experimental environment, followed by evolutionary stasis (or near stasis)."
But, as reported in a later paper (Elena, Cooper and Lenski, 1996), they then
sampled each population at a much finer scale—every 100 generations for the first
3000 generations of the experiment. Now they found clear evidence of a
punctuational "step pattern" (see Fig. 9-11, discussed previously) within the initial
phase of rapid increase that they had previously fit with a simple hyperbolic model.
The authors noted: "When mean fitness was measured every 100 generations over
the period of faster change ... a step function model, in which periods of stasis were
interrupted by episodes of rapid change, gave a better fit to the data than did the
hyperbolic model. Evidently,
to a different sugar (maltose), some populations flourished, but others grew poorly. After
1000 generations on maltose, all twelve populations did improve in fitness, but not nearly so
much (and, more importantly, not nearly so consistently) as on glucose. The starting ge-
notype for the 12 populations had been identical for the first experiment with glucose, but
different (after 2000 generations of evolution on glucose for each population) for the initia-
tion of the subsequent maltose experiment. Apparently, any departure from simple and
controlled experimental conditions towards the genetic and environmental variation in-
variably encountered in the natural world greatly decreases the predictability, while empha-
sizing the contingency, of outcomes.