Punctuated Equilibrium and the Validation of Macroevolutionary Theory 751
stasis and abrupt appearance inheres, without conscious intent or formulation, in
methods developed by the people who use fossils in their daily, practical work.
Evolutionary theory may be a wonderful intellectual frill, but workaday
paleontology, until very recently, used fossils primarily in the immensely useful
activity (in mining, mapping, finding oil, etc.) of dating rocks and determining
their stratigraphic sequence. These practical paleontologists dared not be wrong in
setting their criteria for designating ages and environments. They had to develop
the most precise system that empirical recognition could supply for specifying the
age of a stratum; they could not let theory dictate a fancy expectation unsupported
by observation. Whom would you hire if you wanted to build a bridge across your
local stream—the mason with a hundred spans to his credit, or the abstract
geometer who has never left his ivory tower? When in doubt, trust the practitioner.
If most fossil species changed gradually during their geological lifetimes,
biostratigraphers would have codified "stage of evolution" as the primary criterion
for dating by fossils. In a world dominated by gradualism, maximal resolution
would be obtained by specifying a precise stratigraphic position within a
continuum of steady change, and much information would be lost by listing only
the general name of a species rather than its immediate state within a smooth
transition. But, in fact, biostratigraphers treat species as stable entities throughout
their documented ranges—because the vast majority so appears in the empirical
record. Finer resolution can then be obtained by two major strategies: first, by
identifying species with unusually short durations, but wide geographic spread (so-
called "index fossils"); and, second, by documenting the differing ranges of many
species within a fauna and then using the principle of "overlapping range zones" to
designate geological moments of joint occurrence for several taxa (see Fig. 9-1).