Punctuated Equilibrium and the Validation of Macroevolutionary Theory 897
of a clade by some measure of central tendency, or some salient extreme that
catches our fancy, and then plotting the trajectory of this archetypal value through
time.
LIFE ITSELF. In popular descriptions of evolution, from media to museum
halls, but also in most technical sources, from textbook pedagogy to monographic
research, we have presented the history of life as a sequence of increasing
complexity, with an initial chapter on unicellular organisms and a final chapter on
the evolution of hominids. I do not deny that such a device captures something
about evolution. That is, the sequence of bacterium, jellyfish, trilobite, eurypterid,
fish, dinosaur, mammoth, and human does, I suppose, express "the temporal
history of the most complex creature" in a rough and rather anthropomorphic
perspective. (Needless to say, such a sequence doesn't even come close to
representing a system of direct phylogenetic filiation.)
But can such a sequence represent the history of life, or even stand as a
surrogate either for the fundamental feature of that history, or for the central causal
processes of evolutionary change? When we shift our focus to the full range of
life's diversity, rather than the upper terminus alone, we immediately grasp the
treacherous limitations imposed by misreading the history of an extreme value as
the epitome of an entire system. The sequence of increasingly right-skewed
distributions with a constant modal value firmly centered on bacteria throughout
the history of life (Fig. 9-29) represents only a cartoon or
9 - 29. This cartoon of changing form and range in the histogram of complexity through life's
phylogeny illustrates how we fall into error when we treat extreme values as surrogates or
epitomes of entire systems. A view that emphasizes speciation and diversity might recognize the
constancy of the bacterial mode as the outstanding feature of life's history. From Gould, 1996a.