Punctuated Equilibrium and the Validation of Macroevolutionary Theory 999
not much different than when it first appeared. Another species, related but
distinctly different—"fully formed"—would take its place, persist with little
change, and disappear equally abruptly. Suppose, Eldredge and Gould argued,
that these long periods of no change ("stasis" is the word they use) punctuated
by gaps are not flaws in the record but are the record, the evidence of what
really happens.
How could it be that a new species would make such a sudden appearance?
They found their answer in the model of allopatric speciation. If new species
formed principally in small populations on the geographic periphery of the
range of the species, if speciation occurred rapidly (by rapidly, paleontologists
mean in thousands rather than millions of years), and if the new species then
out competed the old one, taking over its geographic range, the resulting fossil
pattern would be the one observed...
As the new model has become more fully developed, particularly by Steven
M. Stanley of Johns Hopkins (also a paleontologist), it has become more
radical. Its proponents now argue that not only is cladogenesis the principal
mode of evolutionary change (as Mayr stated some 30 years ago) but that
natural selection occurs among species as well as among individuals.... In this
new formulation, species take the place of individuals, and speciation and
extinction substitute for birth and death. In short, there are two mechanisms of
evolution, according to this proposal: in one, natural selection acts on the
individual, and in the other, it acts on the species.
Will the punctuated equilibrium model be assimilated into the synthetic
theory? Or will some radical new concept of evolutionary mechanisms spread
through the scientific strata, out competing the old ideas? At this writing, it is
too early to tell. All that is clear is that this proposal has stimulated a vigorous
debate, a reexamination of evolutionary mechanisms as currently understood,
and a reappraisal of the evidence. All of this indicates that evolutionary
biology is alive and well and that scientist are doing what they are supposed to
be doing—asking questions. Darwin, we think, would have been delighted.THE PERSONAL ASPECT OF PROFESSIONAL REACTIONAmong false dichotomies, the strict division of a professional's reaction into scientific
conclusions based on legitimate judgment and personal reasons rooted in emotional
feelings represent a particularly naive and misleading parsing of human motivation.
Our analytic schemes do require some heuristic divisions, but the notion that good
reason stands in primal antithesis to bad feelings surely caricatures the depth and
complexity of human reactions. All scientific critiques arise in concert with a
complex and often unconscious range of emotional responses (not to mention a social
and cultural context, which scientists, trained to absorb the myth of objectivity, are
particularly disinclined to recognize). The fact that we can analyze the pure logic of
an argument