Punctuated Equilibrium and the Validation of Macroevolutionary Theory 831
asexual protist "species" as the expected consequence of punctuational clone
selection, and therefore a proper analog of gradual trends in metazoan lineages that
arise from cumulated punctuational speciation (see pp. 807-810). For mammals, I
suspect that published reports follow traditions of work and expectation more
closely than they record actual relative frequencies of nature.
I certainly accept the numerous cases of well-documented gradualism in
foraminiferal lineages, and I acknowledge MacLeod's argument (1991) that abrupt
transitions without branching in some sequences (the "punctuated anagenesis" of
Malmgren et al., 1983) may arise as artifacts of condensed intervals of
sedimentation within truly gradualistic trends. But punctuated equilibrium has also
been demonstrated with data of equal abundance and completeness (the elegant
case of Wei and Kennett, 1988, stands out for thorough documentation of both the
geography of origin and subsequent history of nontrending in Globorotalia
(Globoconella) pliozea), and we have little idea, and no firm data, about overall
relative frequencies for tempo and mode of evolution in this group.
Stasis has been demonstrated in other microfossil "species" with equally
dense documentation—e.g., Nichols (1982) on lower Tertiary pollen, Wiggins
(1986) on upper Miocene dinoflagellates, and Sorhannus (1990) on the Pliocene
diatom Rhizosolenia praebergonii. Ross (1990) suggested that putative differences
in relative frequencies might be tested by comparing foraminiferal lineages with
microfossils of sexual metazoans preserved in comparable abundance in the same
sediments—and that forams vs. ostracodes might provide a good test. Indeed,
published cases for ostracodes seem to speak strongly for stasis and punctuation as
a predominant pattern (in contrast with foraminiferal data). I have already
discussed Cronin's (1985) work on Cenozoic ostracodes (see p. 827), and now cite
his general conclusion (p. 60):
Morphologic and paleozoogeographic analysis of Cenozoic marine
Ostracoda from the Atlantic, Caribbean, and Pacific indicates that climatic
change modulates evolution by disrupting long-term stasis and catalyzing
speciation during sustained, unidirectional climatic transitions and,
conversely, by maintaining morphologic stasis during rapid, high-frequency
climatic oscillations. In the middle Pliocene, 4 to 3 million years ago, at
least six new species of Puriana suddenly appeared as the Isthmus of
Panama closed, changing oceanographic circulation and global climate.
Since then morphologic stasis has characterized ancestral and descendant
species during many glacial-interglacial cycles.The origin of new species by branching in response to geographic opportunity
(rise of the Panamanian isthmus), rather than by anagenetic gradualism as a
selective consequence of changing environments, matches the predictions of
punctuated equilibrium. Of the contribution made by stasis to this conclusion,
Cronin writes (p. 61): "Morphologic stasis characterizes most shallow marine
ostracodes from the western Atlantic that were subjected to these climatic