Species as Individuals in the Hierarchical Theory of Selection 697
organelles (or at least the mitochondria and chloroplasts that began their
evolutionary history as symbiotic prokaryotes), and sometimes tissues and modules
of embryological development, can also act (in principle) as suborganismal units of
selection. I make the amalgamation because this level has been largely suppressed,
and therefore doesn't often come to our attention in studies of modern multicellular
organisms. In so doing, I feel some allegiance to the folk taxonomist who (as so
often recorded for indigenous cultures) assiduously names each species (much as a
trained Linnaean systematist would do) for creatures important to his life, but then
lumps into large categories (weeds, butterflies, bugs) the organisms of no great
moment in his world.
As the central premise of his fascinating and seminal book, Buss (1987)
argues that the multicellular individual arose by "the interplay between selection at
the level of the individual and selection at the level of the cell lineage" (p. 29).
More specifically, he attributes the distinctive features of metazoan development to
an initial competition among cell lineages, eventually tamed and regulated by
organismic selection in the interests of bodily integrity. Buss writes: "The thesis
developed here is that the complex interdependent processes which we refer to as
development are reflections of ancient interactions between cell lineages in their
quest for increased replication. Those variants which had a synergistic effect and
those variants which acted to limit subsequent conflicts are seen today as patterns
in metazoan cleavage, gastrulation, mosaicism, and epigenesis" (p. 29).
Clearly, such a concept becomes intelligible only under the aegis of a
hierarchical model of selection, as defended in this book's central thesis. Buss
recognizes this conceptual link, of course, and his work becomes a strong
confirmation of both the efficacy and necessity of this basic reconstruction in
evolutionary theory. In terms similar to the views expressed here, Buss writes (pp.
5 - 6): "The logical structure of Darwin's argument allows any unit to evolve if it
replicates with high fidelity, and if selection distinguishes between the variants.
Species, populations, and lineages of individuals, cells, organelles, and gene
sequences can all potentially evolve. Yet we have been largely content to attribute
the whole of biological diversity to selection upon individuals [organisms]. The
once comfortable cloak of the Modern Synthesis has become restrictive." (I am
also grateful to Buss for recognizing the role of my profession, particularly in the
work of Eldredge, Jablonski, Stanley, Vrba, and myself, in developing the
hierarchical theory of selection. He writes (p. ix): "Indeed, hierarchical
perspectives on evolution are undergoing a rebirth among paleontologists at the
moment.")
In Buss's model of historical and sequential construction for nature's
hierarchy, new levels arise to enclose the individuals of older levels by a two-step
process. The initial features of the nascent level must originate in synergism, or
positive interaction, with selection at the level just below, which formerly stood
topmost, but will now be superseded (in the literal sense of "sat upon") by the
newly emerging style of organization. New levels must begin with such a helpful
boost, for the initial tentative and unformed steps cannot yet possess enough power
to suppress or regulate a well-established level beneath. But