826 THE STRUCTURE OF EVOLUTIONARY THEORY
This case becomes particularly interesting, and merits consideration here, as a
demonstration of how far reliable inference can extend, even when the tempo and
mode of origin for a descendant species cannot be directly resolved (the usual
situation in paleontology). The potential descendant, D. favrina, enters the section
near the top and overlaps in range with D. subucula, thus implying cladogenetic
origin rather than anagenesis. The descendant's larger size and hypermorphic
morphology suggest a simple heterochronic mechanism for the production of all
major differences, hence increasing our confidence in (although clearly not
proving) a hypothesis of direct evolutionary filiation. Finally, the fact that no
morphological differentias of the species undergo any phyletic transformation
within the lifetime of the putative ancestor further underscores the punctuational
character of the transition, whatever the mode followed. The one character that
does change during the tenure of D. subucula (perhaps only ecophenotypically, as
discussed above) does not move towards the morphology of descendant D. favrina.
The authors conclude (1985, pp. 36-37):
Clearly the sedimentary record is complete enough and represents a
sufficiently long period of time to be able to detect phyletic gradualism. Yet
throughout this period D. subucula remains otherwise morphologically
static. Characters that have been modified in closely related species show
no evidence of undergoing gradual transformation within the duration of
the species ... The overlapping ranges of the two species and the total
absence of phyletic gradualism in the characters that serve to distinguish
the species suggests that punctuated equilibrium is a better model for
speciation in this particular case.In a later section (pp. 854-874), I shall discuss the generality of stasis within
taxa or times under the more appropriate heading of empirical work on relative
frequencies. But I shall also note this broader argument here, and in passing, if
only to underscore the strong psychological bias that still pervades the field,
thereby conveying a widespread impression that gradualism maintains a roughly
equal relative frequency with punctuated equilibrium, whereas I would argue that,
in most faunas, only a small minority of cases (surely a good deal less than 10
percent in my judgment) show evidence of gradualism. Under this largely
unconscious bias, most researchers still single out rare cases of apparent
gradualism for explicit study, while bypassing apparently static lineages as less
interesting.
Johnson (1985), for example, studied 34 European Jurassic scallop species,
and concluded (p. 91): "One case... was discovered where... the sudden
appearance of a descendant form could fairly be ascribed to rapid evolution (within
no more than one million years). Inconclusive evidence of gradual change over
some 25 million years was discovered in one of the other lineages studied... but
in the remaining 32 lineages morphology appears often to have been static." Yet
Johnson virtually confines his biometrical study to the two cases of putative
change, presenting only a single figure for just one of the