Punctuated Equilibrium and the Validation of Macroevolutionary Theory 807
of organisms for every tiny subsection of a geographic range. How do new
populations become isolated? How do favorable (or, for that matter, neutral) traits
ever spread through populations so extensive in both space and number?
(2) Morphology and definition. If metazoan stasis can be attributed, at least in
part, to developmental buffering, what (if any) corresponding phenomenon can
keep the phenotypes of simple unicells stable? Perhaps foraminiferal phenotypes
manifest substantial plasticity for shaping by forces of temperature, salinity, etc., in
surrounding water masses (see Greiner, 1974)— as D'Arcy Thompson (1917,
1942) proposed for most of nature in his wonderfully iconoclastic classic, On
Growth and Form—see pp. 1179-1208. (Thompson's claim that physical forces
shape organisms directly holds limited validity for complex and internally buffered
multicellular forms, but his views may not be so implausible for several features of
simpler unicells.) Could many examples of foraminiferal gradualism (compared
with metazoan stasis in similar circumstances) reflect the plasticity of these protists
in the face of gradual changes in the physical properties of enveloping oceanic
water masses through time? If so, such gradual trends would not be recording
evolutionary change in the usual genetic sense.
(3) Most interestingly (as a potential illustration of the main theoretical
concern of this book), we must consider the potential for strongly allometric
scaling of effects from a defined locus of change to other levels of an evolutionary
hierarchy. To reiterate a claim that runs, almost like a mantra, throughout this text:
punctuated equilibrium is a particular theory about a definite level of organization
at a specified scale of time: the origin and deployment of species in geological
perspective. The punctuational character of such change does not imply—and may
even, in certain extrapolations to other scales, explicitly deny—a pervasive
punctuational style for all change at any level or scale. In particular, punctuated
equilibrium posits that tolerably gradual trends in the overall history of phenotypes
within major lineages and clades (including such traditional tales as augmenting
body size in hominids, increasing sutural complexity in ammonoids, or symmetry
of the cup in crinoids) should reveal a punctuational fine texture when placed
"under the microscope" of dissection to visualize the individual (speciational)
"building blocks" of the totality—what we have long called the "climbing up a
staircase" rather than the "rolling a ball up an inclined plane" model of fine
structure for trends.
Similarly, in asking about evolutionary causality under selective models (see
Chapter 8), we need to identify the primary locus of Darwinian individuality for
the causal agents of any particular process—for only properly defined Darwinian
individuals can operate as "interactors" in a selective process: that is, can interact
with environments in such a way that their own genetic material becomes plurified
in future generations because certain distinctive properties confer emergent fitness
upon the individual in its "struggle for existence" (see pp. 656-667). Punctuated
equilibrium maintains that species, as well-defined Darwinian individuals, hold
this causal status as irreducible