702 THE STRUCTURE OF EVOLUTIONARY THEORY
ever since Darwin himself (see Chapter 2, pp. 133-136)—that seem, prima facie,
difficult to explain as beneficial to organisms, but can easily be construed as
valuable for groups. But we should recognize such restricted invocation (only for
cases that trouble organismic traditionalists) as yet another parochial limitation,
and we should acknowledge a potentially general role for interdemic selection
within any species of appropriate population structure. (Under such a criterion of
judgment by relative frequency, we must ask a different, and quite unanswered,
fundamental question: how many higher taxa generally maintain population
structures that promote interdemic selection; in what environments; and with what
correlations to such factors as phylogenetic status, body size, behavioral
complexity, etc.)
If various arguments for the rarity of extensive evolution within large
panmictic populations hold merit, and if Sewall Wright's shifting balance theory
applies to a high percentage of populations, then interdemic selection may become
a major mechanism for evolution within species through time. However, if
punctuated equilibrium generally holds (see Chapter 9 for a defense of this view),
then anagenesis within species will be rare in any case (whether by transformation
via organismic selection under panmixia, or by shifting balance via interdemic
selection in appropriately subdivided populations). Or perhaps, as an intermediate
position, panmictic transformation is rare, but shifting balance frequent, in species
that meet the criteria for appropriate population structure. The high relative
frequency of punctuated equilibrium would then measure the relative rarity of such
population structures, and the few groups that show extensive gradualism within
species may generally subdivide their populations according to Wrightian criteria.
This conjecture has not been tested, but could be, and with an interesting mixture
of paleontological data on the history of species and neontological information on
population structures within modern representatives of the same groups.
In any case, even if Wright's criteria don't hold often enough within the
central range of species during the heart of their geological life, Mayr's peripatric
model of speciation suggests that the origin of most species may occur by a
process close to interdemic selection, and operating near a blurred borderline with
species selection. If many species spawn large numbers of peripherally isolated
demes, but only a few of these demes become species; and if the small class of
successful speciators possess traits at the population level that encourage full
speciation in interaction with the environment; then species will arise by selection
and differential preservation on a just a few "winners" within a set of populations
that begin as demes of an ancestral species (as best illustrated by the probable main
reason for failure of others to speciate— reincorporation of a peripherally isolated
deme into the larger parental population).
For all these reasons, I suspect that selection among deme-individuals holds an
importance as yet unrealized (and perhaps occurring in modes as yet
unconceptualized) within our general picture of evolution. I have, in my career,
witnessed three examples of widespread dismissal by ridicule as part of a
professional ethos: the rejection of continental drift as physically inconceivable;