The Structure of Evolutionary Theory

(Michael S) #1

Punctuated Equilibrium and the Validation of Macroevolutionary Theory 961


Dutch petrologist E. Den Tex (1990), in summing up his career of study on the
nature of granites, credited punctuated equilibrium with reorganizing his lifelong
attempt to make sense of a complicated history, stretching back to the late 18th
century (the neptunist-plutonist debate between the schools of Werner and Hutton),
and featuring fluctuations between poles of two dichotomies (sedimentary vs.
igneous formation, and recruitment from deep magmas vs. transformation from
existing crustal rocks), complicated by shifting allegiances and amalgamations,
followed by breakages, of separable aspects of all end-member theories.
In his fascinating and highly personal paper, entitled "Punctuated equilibria
between rival concepts of granite genesis," Den Tex (1990) notes that he had first
tried to apply other models of noncontinuous and progressive change, especially
the celebrated Hegelian notion of successive syntheses reached by opposition
between a thesis and its antithesis. He then found a better general description, with
new and fruitful hints for explanation, in our model of punctuated equilibrium,
particularly in the parsing of history as discrete steps, analogous to individuated
species with definite sets of properties—a process "in dynamic equilibrium ...
punctuated from time to time by allopatric speciation, i.e. by rapid, random,
discrete steps taking place in locations isolated from the main stem" (1990, p. 216).
Finally, since punctuated equilibrium arose as a theory about change in the
natural world (not in the history of human understanding thereof), Eldredge and I
have been gratified by the utility of our theory in suggesting structurally
homologous modes of change in other branches of natural science. I have been
especially pleased by geological examples distant from our own paleontological
concerns, because no other field can match anglophonic geology— resulting
largely from the legacy of Lyellian uniformitarianism (see Chapter 6)—in explicit,
and often exclusive, fealty to strictly gradualistic models.
Lawless (1998, and a good name for iconoclasts), in an article entitled
"Punctuated equilibrium and paleohydrology," notes the hold that gradualistic
models have imposed on the history of ideas about hydrothermal ore deposits,
particularly of gold. He begins by expressing a paradox: if ores accumulate
gradually in such systems, and given the average amount of gold carried in most
percolating waters, minable deposits should be much more common—indeed
almost ubiquitous in hydrothermal systems that persist for at least 25,000 years.
But the much rarer distribution of such deposits suggests to Lawless that periods of
accumulation must be limited to brief episodes "which cause vigorous boiling
through a restricted volume of the reservoir" (p. 165). Lawless views the general
history of hydrothermal systems—including the development of ore deposits as
just one feature among many—as punctuational, and caused by rapid, intense,
rarely-acting forces: "Such disturbances caused by tectonic activity, magmatism,
volcanic activity, erosion, climatic changes or other processes may occur at long
intervals, but be responsible for producing some of the most significant
characteristics of the system, including economic mineral deposits."
As a matter of potential practical importance, Lawless recognizes that, just

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