Punctuated Equilibrium and the Validation of Macroevolutionary Theory 809
their evolution from our own biased perspective of species, and see their
gradualism as truly phyletic—while it is really the clonal analog of a
gradual evolutionary trend produced by punctuated equilibria and species
selection.Lenski's remarkable studies on controlled evolution of bacteria under
laboratory conditions of replication provide striking evidence for this claim (see
full discussion of this work on pp. 931-936). Lenski and colleagues (Lenski and
Travisano, 1994; Papadopoulos et al., 1999) monitored average cell size for 10,000
generations in 12 lineages of E. colt. Cell size increased asymptotically in each
lineage, steadily for the first 3000 generations or so, but remaining relatively stable
thereafter. The fine structure of increase, however, proceeded in a punctuational
manner in each lineage—a step-like pattern of stability in average cell size,
followed by rapid ratcheting of the full population up to larger dimensions. This
punctuational pattern presumably occurred because clones act as primary
Darwinian individuals in this system. The full lineage must "wait" for sudden
introduction of favorable variation in the form of occasional mutations, initiating
novel clones that can then sweep through the entire lineage to yield a punctuational
step in the overall phylogeny (at a scale of 10,000 generations in phenotypic
history). Predictable, replicable size increase occurs by punctuational clone
selection in each case (see Fig. 9-11). Lenski's powerful result does not illustrate a
case of punctuated equilibrium, sensu stricto, but he does provide a challenging
and instructive
9 - 10. The supposed gradualism noted in many foram species may represent a view, from too
high a level, of an overall trend within a phyletic sequence properly analyzed in terms of
punctuational events at the level of clone selection—the appropriate mode in such asexual forms.
1 shows a conventional metazoan lineage in punctuated equilibrium. 2 shows the apparent
gradualism in a foram lineage. 3 shows a gradualistic segment between B and C magnified so
that the appropriate process of punctuational clone selection becomes visible. From Gould and
Eldredge, 1977.