946 THE STRUCTURE OF EVOLUTIONARY THEORY
unattainability and adaptive inviability of intermediates, and therefore envisage a
one-step transition (with substantial opportunity, perhaps, for later adaptive
finetuning).
In favor of punctuational origins (rapid transition between domains, based on
structural properties of endpoints as well coordinated states that actively resist
change, and with intermediary forms as unstable, and "driven" towards one of the
endpoints) versus saltational events (truly sudden transition, scaled to the
magnitude of the unit of change and enforced by the absolute structural
inaccessibility of intermediary states), Blackburn cites several features of squamate
viviparity. The putative intermediary stage of "prolonged oviparous egg retention"
(p. 206), while empirically rare, structurally unstable, and adaptively compromised
(as discussed above), does exist as the characteristic form of a few species in
nature, not just as a facultative or transient state of a population in transformation.
As the bimodality of Figure 9-35 shows, prolonged oviparous egg retention does
represent an attainable intermediary state between two endpoints. But few forms
occupy this uncertain ground between advantageous configurations.
In a second indication of punctuational rather than saltational change,
Blackburn notes that a viable first step to intermediacy does exist in nature as a
strategy that can be activated under certain environmental conditions: "facultative
egg-retention with continued intraoviductal development" (p. 206). Several
squamate species exhibit this phenotypic flexibility in development. "Such
facultative retention could provide raw material for natural selection, making more
likely the evolution of a pattern in which prolonged egg-retention was obligative."
As a third confirmed prediction, favoring punctuation over saltation when
joined with the two previous arguments (but unable to make the distinction
otherwise, while confuting gradualism in any case), the phenotypic similarity of
oviparous and viviparous congeners affirms the relative ease and accessibility of
transition: "As an evolutionary unstable pattern, prolonged egg-retention might
lead to viviparity, or might revert to typical oviparity; thus the less genotypic
change involved, the more probable the origin of viviparity would be" (p. 206).
Blackburn's final paragraph (p. 212) serves as an apt reminder about the
restrictive nature of gradualistic bias, and of the power and inherent probability of
punctuational alternatives in a world that may favor this mode of change as a
general structural property of material organization at all scales: "For over 60
years, research on amniotes has assumed that gradualistic change is the sole
mechanism by which viviparity, placentation, and placentotrophy could have
evolved. Future empirical and theoretical analyses of reproduction in squamates
and other vertebrates should not overlook the potential of non-gradualistic models
as explanations for evolutionary change and the biological patterns it has
produced."
PUNCTUATIONAL ANALOGS IN FAUNAS AND ECOSYSTEMS. If stable locations,
reached by rapid movement though "perilous" intermediary terrain, sets the
structural basis of punctuational change for the "internalities"