Darwin's Dangerous Idea - Evolution and the Meaning of Life

294 BULLY FOR BRONTOSAURUS Punctuated Equilibrium: A Hopeful Monster 295
and west as it goes, crossing in the Bering Strait region, and perhaps even
encircling the globe like the herring gulls. Then, as the ice advances south
during the next ice age, it sheers off the connections between the Asian and
North American parts of the family, creating two isolated ranges that then
naturally diverge into distinct species, but as they both move southward in
their respective hemispheres, they continue to look much the same, because
they track their favored climatic conditions, instead of staying put and going
in for further winter adaptations. 6
Another possible explanation of punctuated equilibrium is purely theoretical.
Stuart Kauffman and his colleagues have produced computer models
that exhibit behavior in which relatively long periods of stasis are interrupted
by brief periods of change not triggered by any "outside" interference, so this
pattern seems to be an endogenous or internal feature of the operation of
particular sorts of evolutionary algorithms. (For a recent discussion, see Bak,
Flyvbjerg, and Sneppen 1994.)
It is quite clear, then, that equilibrium is no more a problem for the neo-
Darwinian than punctuation; it can be accounted for, and even predicted. But
Gould has seen yet another revolution lurking in punctuated equilibrium.
Maybe the horizontal steps of punctuation are not just (relatively) rapid steps
in Design Space; maybe what is important about them is that they are steps of
speciation. How could this make a difference? Look at figure 10.10.
In both cases, the lineage at K got where it got by exactly the same
sequence of punctuations and equilibria, but the case illustrated on the left
shows a single species undergoing rapid periods of change followed by long
periods of stasis. Such change without speciation is known as anagenesis.
The case illustrated on the right is an instance of cladogenesis, change via
speciation. Gould claims that the rightward trend in the two cases would have
a different explanation. But how could this be true? Recall what we
learned in chapter 4: speciation is an event that can only be retrospectively
identified. Nothing that happens during the sideways move could distinguish
an anagenetic process from a cladogenetic process. There has been speciation
only if there is a later flourishing of separate branches that survive long
enough to be identifiable as separate species.
Couldn't there be special processes of what we might call hopeful speciation—or
incipient speciation? Consider a case in which speciation does
occur. Parent-species A splits into daughter-species B and C.
Now wind back the tape just far enough in time to drop a bomb (an
asteroid, a tidal wave, a drought, poison ) on the earliest members of the B
species, as in the middle diagram. Doing this turns what had been a case of
speciation into something indistinguishable from a case of anagenesis (on the
right). The fact that the bomb prevents those whose offspring it kills
6. George Williams (1992, p. 130) disputes the importance of habitat tracking in stasis,
noting that parasites, "seasonal amplitude of insolation" (amount of sunshine), and many
other environmental factors would always be different after a geographical move, so that
populations would never be able to stay in exactly the same selective environment, and
hence would be subjected to selection pressure in spite of moving. But it seems to me
that much if not all of the adjustment to these selection pressures could be invisible to
paleontology, which can only see in the fossil record the preserved changes in hard-part
design. Habitat tracking could be responsible for much of the paleontologically observable
stasis (and what other stasis do we know about?), even if Williams is right that this
body-plan stasis would have to mask concurrent nonstasis at most if not all other design
levels in response to the many environmental changes that would have to accompany any
long-range habitat-tracking moves. And unless many species moved in unison in their
habitat tracking, there couldn't be habitat tracking at all, since other species are such
crucial elements in any species' selective environment.