Punctuated Equilibrium and the Validation of Macroevolutionary Theory 879
proposals, although different in implications about styles and reasons for limited
change in species, remain primarily complementary in their common attribution of
stasis to reasons based on satisfactory current status—the first on immediate
optimality of overt features, the second on inherent plasticity within a current, and
presumably adaptive, norm of reaction.)
Wake et al. (1983), for example, document how salamanders, artificially
raised to encounter only fixed potential prey, will learn to eat immobile objects,
thus contradicting "the widespread assumption that amphibians feed only on
moving prey" (p. 216), and also permitting substantial "adaptation" (physiologist's
sense again) to feeding regimes without disturbing the stasis of evolved form. In a
thought-provoking conclusion, Wake et al. (1983, p. 219) site (and cite) stasis as
one component of a more general attitude towards stability of systems and
preference for non-change, with evolution conceptualized as a "default option" in
the history of life—in contrast with the usual view of active and normative change
embodied in the first explanation of stabilizing selection:
Stasis is but the most rigid form of the stability that pervades living
systems. Thus organisms have evolved as systems resistant to change, even
genetic change. While changing environmental conditions may ultimately
necessitate change in the system, until some critical point the system
remains stable and compensating. The living system is sometimes
envisioned metaphorically as a kind of puppet, with enormous numbers of
strings, each controlled genetically, or as a blob of putty that can flow in
any direction given sufficient force (selection). Our metaphor is the living
system as a balloon, with the environment impinging as countless blunt
probes. The system compensates environmental and genetic changes, and
persists by evolving minimally.DEVELOPMENTAL CONSTRAINT. This proposal veers more towards heterodoxy
in ascribing stasis to an internally specified inability to change (thereby implying
frequent suboptimality of adaptation), rather than to lack of adaptive impetus for
change due to current optimality (explanation one) or flexibility within a current
constitution (explanation two). (This notion of inability stands forth most clearly in
the strict definition—too strict in my view (see Chapter 10)—of constraint as
absence of genetic variation for a particular and potentially useful alteration, as in
the consensus concept of Maynard Smith et al., 1985.)
In our original paper on punctuated equilibrium, Eldredge and I (1972),
basing our arguments partly on Mayr's (1954, 1963) concept of genetic revolutions
in speciation of peripherally isolated populations, but more on Lerner's notions
(1954) of ontogenetic or developmental, but especially of genetic, "homeostasis,"
proposed such constraint as the primary reason for stasis. We wrote (1972, pp.
114 - 115):
If we view a species as a set of subpopulations, all ready and able to
differentiate but held in check only by the rein of gene flow, then the
stability