712 THE STRUCTURE OF EVOLUTIONARY THEORY
fallacy in denying status to species selection by confusing personal interest with
general importance.
Only one line of defense remains open to those who still wish to deny the
importance of species-level processes after correcting this psychological fallacy,
and admitting that trends and changing patterns in diversity rank as vital subjects in
a complete evolutionary theory, and also represent "what species do." Such a
Darwinian stalwart must argue that all (or nearly all) phenomena at the species
level find their causes in upward translation from ordinary natural selection on
organisms. Thus, if current biotas feature half a million species of beetles, this
plethora can only imply that beetle organisms maintain a particularly favorable
adaptive design. And if geological trends privilege increasing body size, larger
brains, more complex ammonite sutures, more symmetrical crinoid cups, fewer
horse toes, and a thousand other documented patterns, these features must triumph
by their adaptive value to organisms. I shall make no further arguments against
such a narrow perspective here (to save my rebuttal for Chapter 9, pp. 886-893),
and will only quote a great American character, Sportin' Life in Porgy and Bess, to
remind us that received wisdom does not always prevail:
The things that you're liable to read in the Bible
It ain't necessarily so.THE CLADE-INDIVIDUAL Although a logical space must exist in our structure
of explanation for this highest level of the evolutionary hierarchy, I am not sure
that clade selection plays a major role in evolution. Most clades contain so few
parts (species) that their waxing and waning must often occur by processes that
either operate as random inputs to the clade level, or result from selection among
subparts (species selection, or lower-level selection), and therefore appear as drives
at the clade level (and not as selection among entire clades treated as individuals).
Secondly, while I have advocated a plurality of mechanisms for coherence of
individuals at various levels in the hierarchy, I do have trouble in conceptualizing
an adequate "glue" for clades, especially since their parts (species) may live in such
complete independence, and in such different ecologies, on distant continents.
Finally, clades maintain the peculiar property (perhaps only an odd "allometric"
consequence of necessary structure at this highest level, and not any compromise
in efficacy) of necessarily originating as a single subpart—the founding species,
and gaining definition (as a full level) only retrospectively, after adding new parts
(more species) sequentially.
How then, given all these difficulties, could clades compete, qua clades as
discrete and integral evolutionary items, even under the broad definition (see p.
706) that does not require direct contact or even life in sympatry? Is a clade,
uniquely among evolutionary individuals of the hierarchy, more a "holding firm"
for subparts than a coherent entity frequently subject to selection at its own level?
One route to claiming a potential importance for clade selection remains