The Modern Synthesis as a Limited Consensus 527
invisibility (see Chapter 10, pp. 1175-1178 for the modern relevance and refutation
of this striking image).
Dobzhansky poses the key question of organic form and taxonomy: why do
organisms form discrete and clearly nonrandom "clumps" in populating
morphological space? Why does the domain of mammalian carnivores contain a
large cluster of cats, another of dogs, a third of bears, leaving so much unoccupied
morphological space between? Dobzhansky begins by "promoting" Wright's model
of the "adaptive landscape" to an inappropriate level. In so doing, Dobzhansky
subtly shifts the model's meaning from an explanation for nonoptimality (with
important aspects of nonadaptation) to an adaptationist argument about best
solutions. Wright devised his model to explain differentiation among demes within
a species. He proposed the metaphorical landscape to justify a fundamentally
nonadaptationist claim: If a "best solution" exists for the phenotype of a species
(the highest peak in the landscape), why don't all demes reside there? But if we
"upgrade" the model to encompass differences between species within a clade, then
metaphorical landscapes mutate into a framework for strict adaptationism. Each
peak now becomes the optimal form for a single species (not the nonoptimal form
for some demes within a species). And related peaks represent a set of best
solutions as the various adaptations of separate evolutionary entities within a clade.
Dobzhansky then attempts to solve the problem of clumping with an
adaptationist argument based upon the organization of ecological space into
preexisting optimal "places" where good design may find a successful home.
Evolution has produced a cluster of cats because an "adaptive range," studded with
adjacent peaks, exists in the economy of nature, waiting, if you will, for creatures
to move in. In other words, discontinuity in taxonomic space maps discontinuity in
optimal form for available environments, with adaptation as the agent for mapping.
The enormous diversity of organisms may be envisaged as correlated with
the immense variety of environments and of ecological niches, which exist
on earth. But the variety of ecological niches is not only immense, it is also
discontinuous... The adaptive peaks and valleys are not interspersed at
random. "Adjacent" adaptive peaks are arranged in groups, which may be
likened to mountain ranges in which the separate pinnacles are divided by
relatively shallow notches [sic, Dobzhansky does indeed mean "notches" in
this passage, not "niches" (as later in the quotation)]. Thus, the ecological
niche occupied by the species "lion" is relatively much closer to those
occupied by tiger, puma, and leopard than to those occupied by wolf,
coyote, and jackal. The feline adaptive peaks form a group different from
the group of the canine "peaks." But the feline, canine, ursine, musteline,
and certain other groups of peaks form together the adaptive "range" of
carnivores, which is separated by deep adaptive valleys from the "ranges"
of rodents, bats, ungulates, primates, and others. In turn, these "ranges" are
again members of the