more complex design through time--amidst all the fluctuations and
backings and forthings that must characterize a process primarily devoted
to constructing a better fit between organisms and changing local environments.
Darwin certainly thought so when he wrote:
The inhabitants of each successive period in the world's history have
beaten their predecessors in the race for life, and are, insofar, higher
in the scale of nature; and this may account for that vague yet ill-defined
sentiment, felt by many paleontologists, that organization on the whole
has progressed.
I regard the failure to find a clear "vector of progress"
in life's history as the most puzzling fact of the fossil record. But
I also believe that we are now on the verge of a solution, thanks to a
better understanding of evolution in both normal and catastrophic
times. We need a two-tiered explanation for patterns (or nonpatterns)
in the history of life.
I have devoted the last ten years of my professional life in paleontology
to constructing an unorthodox theory for lack of expected patterns during
normal times--the theory of punctuated equilibrium. Niles Eldredge
and I, who must admit responsibility for this particularly uneuphonious
name, argue that the pattern of normal times is not one of continuous
adaptive improvement within lineages. Rather, species form rapidly in
geological perspective (thousands of years) and tend to be highly stable
for millions of years thereafter. Evolutionary success must be assessed
among species themselves, not at the traditional Darwinian level of struggling
organisms within populations. The reasons that species succeed are many
and varied--high rates of speciation and strong resistance to extinction,
for example--and often involve no reference to traditional expectations
for improvement in morphological design. If punctuated equilibrium dominates
the pattern of normal times, then we have come a long way toward understanding
the curiously fluctuating directions of life's history. Until recently,
I suspected that punctuated equilibrium might resolve the dilemma all
by itself.
I now realize that the fluctuating pattern must be constructed by a
complex and fascinating interaction of two distinct tiers of explanation--punctuated
equilibrium for normal times, and the different effects produced by separate
processes of mass extinction. Whatever accumulates by punctuated equilibrium
in normal times can be broken up, dismantled, reset, and dispersed by
mass extinction. If punctuated equilibrium upset traditional expectations,
mass extinction is even worse. Organisms cannot track or anticipate the
environmental triggers of mass extinction. No matter how well they adapt
themselves to environmental ranges of normal times, they must take their
chances in catastrophic moments. And if extinctions can demolish more
than 90 percent of all species, then we must be losing groups forever
as a result of pure bad luck among a few clinging survivors designed for
another world.
Heretofore, we have thrown up our hands in frustration at the lack or
expected pattern in life's history--or we have sought to impose a
pattern that we hoped to find on a world that does not really display
it. Perhaps now we can navigate between a Scylla of despair and a Charybdis
of comforting unreality. If we can develop a general theory of mass extinction,
we may finally understand why life has thwarted our expectations--and
we may even extract an unexpected kind of pattern from apparent chaos.
The fast track of an extraordinary meeting in Indianapolis may be pointing
the way.
Stephen Jay Gould teaches biology, geology, and the history of science
at Harvard University.
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