652 THE STRUCTURE OF EVOLUTIONARY THEORY
trends within a clade. Change must therefore be concentrated in events of
branching speciation, and trends must arise by the differential sorting of species
with favored attributes. If new species generally arise in geological moments, as
the theory of punctuated equilibrium holds, then trends owe their explanation even
more clearly to higher-level sorting among species-individuals acting as discrete
entities with momentary births and stable durations in geological time.
Organismic selection may trump species selection in principle when both
processes operate at maximal efficiency, but if change associated with speciation
operates as "the only game in town," then a weak force prevails while a potentially
stronger force lies dormant. Nuclear bombs certainly make conventional firearms
look risible as instruments of war, but if we choose not to employ the nukes, then
bullets can be devastatingly effective. The empirical pattern of punctuated
equilibrium therefore becomes the factual "weapon" that overcomes Fisher's strong
theoretical objection to the efficacy of species selection.
(This argument provides a second example for the importance of punctuated
equilibrium in validating the independence of macroevolutionary theory by failure
of pure extrapolationism from microevolutionary dynamics. We saw previously
(pp. 604-608) that punctuated equilibrium strongly fosters the argument for species
as evolutionary individuals capable of operating as units of selection. We now note
that punctuated equilibrium also affirms the potential strength of species selection
against a cogent theoretical claim for its impotence.)
In summary, three of four classical arguments against higher-level selection
do not apply to species, while the fourth loses its force in a world dominated by
punctuated equilibrium. I see no barrier to the cardinal importance of species
selection in the history of life.
EMERGENCE AND THE PROPER CRITERION
FOR SPECIES SELECTIONDifferential proliferation or downward effect?
This subject and its literature, as I have noted throughout the chapter, have been
plagued to an unusual degree by conceptual confusions and disputes about basic
definitions and terminology. As an important example, and as many participants
have noted (see especially Damuth and Heisler, 1988; and Brandon, 1988,1990),
two quite different criteria for the definition of higher-level selection have
circulated through the literature. (In most cases, they yield the same conclusion, so
this situation has not produced anarchy; but in a few cases, some crucial, they may
lead to different assertions, so the situation has fostered confusion.)
In the first approach, one chooses a focal level of analysis (conventionally one
of the two lower levels of organism or gene), and then considers the effect of
membership in a higher-level group upon fitness values of the chosen lower-level
unit. If, for an identical organism, life in one kind of deme yields a