Punctuated Equilibrium and the Validation of Macroevolutionary Theory 829
9 - 15. Two hundred thousand years of stasis, during intense climatic fluctuation of Pleistocene ice
cycles, in the ostracod Puriana mesacostalis. From Cronin (1985). The three circles around the
specimens of this species show increased variation with expanding amounts of time: a single
sample representing 100 to 1000 years, one formation encompassing 20,000 to 50,000 years, and
two formations representing 100,000 to 200,000 years. The range of variation expands but the
modal values do not change at all—as a hypothesis of stasis would predict.
have documented extensive and complex temporal variability, both for single
characters and correlated complexes, over tens of thousands of years (the 1985
study, for example, included several sampling pits covering about 1/3 of the total
sedimentary record in a full sequence of approximately 110,000 years). They found
some gradual trends in parts of the sequence, a great deal of fluctuation, and a few
levels of abrupt alteration, often completely reversing a gradual change built
through most of preceding time. In sum, this extensive and multifarious variation
includes no sustained or accomplished directionality, and means for most single
characters end up about where they began, whatever the internal wanderings
between endpoints. Bell et al. (1985, p. 264) conclude: "Despite the temporal
trends and heterogeneity of all characters through time, the end members [oldest
and youngest sample] of only two of the time series (i.e. dorsal spine and dorsal fin
ray numbers)