Punctuated Equilibrium and the Validation of Macroevolutionary Theory 801
death or amalgamation. They produce the ubiquitous and geologically momentary
fluctuations that characterize and embellish the long-term stasis of species. They
are, to use Mandelbrot's famous metaphor for fractals, the squiggles and jiggles on
the coastline of Maine depicted at a scale that measures the distance around every
boulder on every beach along the shore, and not at the resolution properly enjoined
when the entire state appears on a single page in an atlas. Macroevolution
represents the page of the atlas. The distance around each boulder (marking
substantial but ephemeral changes in local populations of guppies and lizards)—
however important in the immediacy of an ecological moment—becomes invisible
and irrelevant (as the transient fluctuations of stasis) in the domain of sustained
macroevolutionary change (Fig. 9-9).
In other words, morphological change correlates so strongly with speciation
not because cladogenesis accelerates evolutionary rates, but rather because such
changes, which can occur at any time in the life of a local population, cannot be
retained (and sufficiently stabilized to participate in selection) without the
protection provided by individuation—and speciation, via reproductive isolation,
represents nature's preeminent mechanism for generating macroevolutionary
individuals. Speciation does not necessarily promote evolutionary change; rather,
speciation "gathers in" and guards’ evolutionary change by locking and
stabilization for sufficient geological time within a Darwinian individual of the
appropriate scale. If a change in a local population does not gain such protection, it
becomes—to borrow Dawkins's metaphor at a macroevolutionary scale—a
transient duststorm in the desert of time, a passing cloud without borders, integrity,
or even the capacity to act as a unit of selection, in the panorama of life's
phylogeny.
To cite Futuyma's summary of his powerful idea (1987, p. 465): "I propose
that because the spatial locations of habitats shift in time, extinction of and
interbreeding among local populations makes much of the geographic
differentiation of populations ephemeral, whereas reproductive isolation confers
sufficient permanence on morphological changes for them to be discerned in