848 THE STRUCTURE OF EVOLUTIONARY THEORY
9 - 20. From Heaton (1993). Data that had, in the past, been interpreted as a gradualistic evolution
of increasing size within a single species actually represent a change in relative abundance of
two species, each stable throughout its interval—with the species of larger body size gradually
becoming more common in the local section.as ancestral I. parvidens declines, throughout the remainder of Orellan times.*
Interestingly, I. typus does undergo a small anagenetic increase following the
extinction of I. parvidens, "but this change is minor and not deserving of
chronospecies recognition" (Heaton, 1993, p. 297), and the species, in any case,
becomes extinct soon thereafter—a common pattern,
- As an example of the conceptual stranglehold that gradualism once imposed upon
such data, the major study done before punctuated equilibrium on the evolution of these
rodents presupposed anagenetic gradualism at a constant rate: "This treatment assumes
that a regular increase in size continued at approximately the same rate throughout
Orellan time" (Howe, 1956, p. 74). When Howe then detected accelerated change at two
paleosols marking boundaries of substages within the Orellan, he assumed (without any
direct evidence) that the paleosols must mark diastems, or time gaps, compressing a true
gradualism of change into the literal appearance of a small hiccup. But Heaton found no
evidence for any temporal hiatus at these boundaries.
Heaton's results included both punctuated equilibrium (two stable species changing
only