Punctuated Equilibrium and the Validation of Macroevolutionary Theory 901
iteration of this sequence (and with a predictable distribution of arrival times)
simply because he cannot penetrate the bar wall and must eventually "reflect off"
whenever he hits this boundary. (Of course, he will also end up in the gutter even if
he moves preferentially towards the bar wall; in this case, the average time of
arrival will be longer, but the result just as inevitable.)
The issue of active drive (a small bias in relative frequency fueled by the
general Darwinian advantages of complexity) vs. passive drift (predictable
movement in a random system based on the model of the drunkard's walk) for the
expansion of the right tail must be resolved empirically. But the macroevolutionary
reformulation of life's history in variational terms establishes a conceptual
framework that permits this question to be asked, or even conceived at all, for the
first time. Initial studies on mammalian vertebrae and teeth, foraminiferal sizes,
and ammonite sutures have been summarized in Gould, 1996a, based on
pioneering studies of McShea, Boyajian, Arnold, and Gingerich. This initial
research has found no departure from the random model, and no overall preference
for increase in complexity in studies that tabulate all events of speciation.
GENERAL RULES. The older literature of paleobiology focused on the
recognition and explanation of supposedly general "rules" or "laws" regulating the
overt phenomenology of life's macroevolutionary pattern. As the modern synthesis
developed its core of Darwinian explanation, several leading theorists (see
especially Haldane, 1932, and Rensch, 1947, 1960) tried to render these laws as
large-scale expressions of evolution's control by adaptive anagenesis in
populations under Darwinian natural selection.
This subject fell out of favor for several reasons, but in large part because
non-adaptationist explanations deemed less interesting (and certainly less
coordinating) than accounts based on natural selection, provided an adequate
compass for most of these "laws." Thus, for example, Dollo's law of irreversibility
9 - 31. The standard statistical model of the "drunkard's walk" shows that even the expanding right
tail of life's right-skewed histogram of complexity may arise within a random system with equal
probabilities for the movement of any descendant towards either greater or lesser complexity.
From Gould, 1966a.