Punctuated Equilibrium and the Validation of Macroevolutionary Theory 849
9 - 21. Note that both species of Ischyromys live sympatrically and remain in stasis in some parts
of their range, particularly in Nebraska and Wyoming.
in relative abundance) and gradualism (minor size increase within the larger species at
the end of its range) in a total pattern, but he concluded (correctly, I think, in my own
biased way) that punctuated equilibrium had shown greater utility in challenging previous
assumptions that had stymied proper conclusions. He closed his paper by writing (1993,
p. 307): "So, Ischyromys displays features of both 'punctuated equilibria' and 'phyletic
gradualism' as defined by Eldredge and Gould (1972). But the primary revelation of this
study is that what was thought to be a single gradually evolving lineage must now be
seen as the replacement of one stable species by another." (In the fairness of full
disclosure, Heaton did his graduate work under my direction. But I really do encourage
independence and contrariness, and some of my students have documented gradualism,
even in their Ph.D. dissertations, when truly (and for the only time) beholden to my
"official" approval—e.g. Arnold, 1982.)