698 THE STRUCTURE OF EVOLUTIONARY THEORY
stabilization of the new level, implying a power to suppress at least some forms of
harmful proliferation from within, then requires negative interaction, once the new
and higher level achieves enough coherence to act in its own right.
Since we have no direct data for key transitions that occurred so long ago and
left no fossil evidence (so far as we know), Buss constructs some hypothetical
examples of how such a process could work. (Such entirely speculative scenarios
must be understood within their acknowledged limits—that is, as hypothetical
stories, "cartoons" in Buss's words, invented to illuminate a potential mode, and
not as claims about any historical actuality.) For example, if the first tentative
multicellular organisms evolved as little more than spherical colonies of identical
protists floating in the ocean, how might essential organismic properties like
cellular differentiation emerge? Suppose that a variant cell lineage arose in such a
loosely knit, hollow sphere of cells, causing members of the new line to enter the
sphere's center, where proliferation could continue. In this way, a new cell lineage
(and the beginning of cellular differentiation for the organism) could originate and
proliferate by selection at the cell level. Buss then supposes that such an event
might also be beneficial for the organism, and he draws an analogy to the ontogeny
of some modern sponges:
The origin of a variant cell line which entered the center of such a sphere to
continue cell division... may have produced a structure which was
sufficiently negatively buoyant to fall to the sea floor. Many modern
sponges ... do just this. A flagellated sphere populated by amoeboid cells
simply drops to the ocean bottom... The pelago-benthic life cycle of
sponges may have arisen as a consequence of variants which, in pursuing
their own replication, fortuitously presented the individual with a benthic
existence and all the attendant opportunities inherent in the invasion of a
new adaptive zone.This move toward a more complex and better-integrated organism begins with
an initial synergism between cellular and organismic selection (origin of a new cell
lineage by invasion and proliferation in the organism's hollow center, leading to
organismal advantages through an imposed change of habitat). But later
stabilization of this innovation requires the suppression of cell lineage selection by
the organismic level—for if the two cell lineages (at the sphere's periphery and
center) engage in an anarchic battle for ever greater representation in cellular
percentages, either the organism will lose coherence and die, or one lineage will
win and the organism will return to its previous state of minimal differentiation.
Moving away from speculation and towards an explanation of metazoan
development, Buss interprets several defining features of many (but not all)
metazoan phyla as records of successful suppression of cell-lineage selection by
organismal selection from above. In particular, he views early germ-line
sequestration (Weismann's crucial criterion in his defense of Darwin against
resurgent late 19th century Lamarckism, see Chapter 3), and maternal
predestination,