Punctuated Equilibrium and the Validation of Macroevolutionary Theory 947
of evolution in complex organic phenotypes, then a similarly episodic, rather than
evenly flowing, mode of change might characterize the "externalities" of
environments that regulate any coordinated evolutionary tempo among
components of biotas and ecosystems. I have already considered the scale of
environmental punctuation most immediately relevant to punctuated equilibrium—
the geological history of regional biotas (see pp. 916-922 on coordinated stasis, the
turnover-pulse hypothesis, and other notions of faunal transition by coincidence of
punctuational extinction and origination in a high percentage of species within a
previously stable biota). But the generality of such punctuational tempos in
external controls might also extend to levels both below and above the direct
mechanics of speciation itself.
In a general argument strikingly similar to Blackburn's for the evolution of
squamate viviparity, Smith (1994) holds that gradualistic assumptions have
stymied our understanding of evolutionary processes at the small scale of
ecological immediacy in deep-sea faunas. No other environment has been so
conventionally associated with plodding, incremental change through substantial
periods of time. Smith begins his article by noting the "the deep-sea floor is
traditionally perceived as a habitat where low food flux and sluggish bottom
currents force life to proceed at slow, steady rates. In this view, benthic community
structure is controlled by equilibrium processes, such as extreme levels of habitat
partitioning, made possible by remarkable ecosystem stability" (Smith, 1994, p. 3).
As indicated by the title of his article—"Tempo and mode in deep-sea benthic
ecology: punctuated equilibrium revisited"—Smith holds that we must revise this
traditional view, and reconceive the deep-sea as a punctuational domain where
"endogenous disturbances may be relatively frequent, and where pulses of food
reach the seafloor from the upper ocean" (p. 3). In what he labels as a "parallel
argument" to our punctuated equilibrium from a much lower scale of analysis—in
other words, as a claim for conceptual homology of constraining structural
principles (in the language of this section)—Smith discusses three examples of
"pulsed events that 'punctuate' the apparent 'equilibrium' of the deep-sea floor" (p.
3), and that "may substantially influence processes of modern and past ecological
significance including (1) maintenance of macrofaunal diversity and population
structure, (2) deposit-feeder-microbe interactions and associated trace production,
and (3) dispersal and biography of chemosynthetic communities at the deep-sea
floor" (p. 3).
First, Smith documents the importance of "pulsed physical disturbance" in
benthic faunas of the Nova Scotian Rise (4750-4950 m)—particularly of erosional
"storms" that scour and redeposit sediments "to depths of millimeters-centimeters
over areas encompassing at least tens of square kilometers" (p. 7), and that strongly
influence both the composition and successional stage of local faunas.
In a second microbiotal example, Smith documents the importance of
"phytodetrital pulses" in nutrition for the deep-sea macrofauna. The "slow and
steady 'drizzle'" usually regarded as the gradual (and meager) planktonic