Species as Individuals in the Hierarchical Theory of Selection 709
Species 3 become extinct, but Species 2 survives thanks to populations on Islands
B and D—that is, only by virtue of populations allopatric with Species 1 and 3.
Clearly, Species 2 has survived as a result of greater geographic range, caused
by whatever organismal, deme, or species traits permitted the colonization of all
islands. Geographic range may be either an emergent or aggregate trait of
successful Species 2; but, in any case, this trait exists at the species level and
confers an irreducible fitness based on superior range (obviously a property of the
species, and not of any individual organism, deme, or avatar). This hypothetical
case presents a potential and plausible example of species selection based on a trait
of the entire species and its complete range— and explicitly not on any sympatric
avatar, or any other subsection of the full entity.
SPECIES SELECTION AS POTENT. TWO separate arguments, one empirical and
the other theoretical; have been raised against the efficacy of species selection. The
first, which I regard as unfair, claims that a paucity of currently recorded empirical
examples must indicate the rarity of the phenomenon. I would respond, first of all,
that a few excellent (and elegant) cases have been well documented, so this process
cannot rank as a distant plausibility waiting for an improbable verification, as some
critics have charged. Jablonski (1987), for example, performed a pioneering study
on species selection in Cretaceous mollusks during the long background interval
preceding the mass extinction at the period's end. He found that species with
planktotrophic larvae (defined as floating and feeding, and therefore remaining
aloft for substantial time) generally have larger geographic ranges and longer
geological durations than species with nonplanktotrophic larvae (defined as either
never planktonic, or floating without feeding, and therefore aloft for only a short
period).
Jablonski supplies good inferential evidence for the two key claims that a
hypothesis of species selection requires. First, he presents a strong case that
geographic range not only correlates with longevity, but also helps to cause the
extended duration. Species tend to reach their maximal range soon after their
origin, and to maintain this breadth thereafter as a potent hedge against extinction.
Second, he calculated a strong heritability for geographic range by assessing the
parent-offspring regression for this character. Geographic range surely constitutes
a character of the species, not (obviously) of individual organisms. This trait
confers an emergent fitness on species that gain increasing longevity thereby. All
necessary attributes for an interpretation based on species selection have therefore
been identified.
(The case also includes interesting complexities. As mentioned previously for
similar examples in Tertiary mollusks, nonplanktotrophic species generally
experience shorter longevity and maintain smaller populations in their more
restricted ranges; but they also speciate more frequently, a presumed consequence
of greater ease in forming isolated populations—for their evolution of larvae
without extensive periods of flotation restricts gene flow among demes. Thus, the
greater longevity of planktotrophic species need not