The Modern Synthesis as a Limited Consensus 559
so new categories gradually arise, first at smaller and then at higher levels" (in Tax
and Callender, 1960, volume 3, p. 127).
In the panel's stated agenda of 16 points, only one even hints at non-adaptive
phenomena, and only as an adjunct to selectionist orthodoxy. Huxley addressed
this topic midway through the discussion: "Natural selection may lead to side
effects, which at the same time are of no adaptive value but may later provide the
basis for adaptive changes" (Tax and Callender, 1960, volume 3, p. 125).
Vertebrate paleontologist E. C. Olson, the symposium's lone (and very gentle)
doubter (see pp. 574-576), ventured a perceptive comment on this point. But his
words, as Moses said of Pharaoh's chariots, promptly "sank as lead in the mighty
waters." The entire discussion of this topic occupies less than a page. Olson said:
"This is the general area in which we can include events that are random with
respect to the adaptive value of the genotype of populations. I refer to the simple
matter of accident—for example, the effects of a forest fire on a population ... This
sort of side effect, the impact of accidents and other factors producing non-
adaptive shifts, may cause very rapid changes and give completely new shape to
the course of evolution. I think this is an extremely important evolutionary factor"
(in Tax and Callender, 1960, volume 3, p. 125).
If the Synthesis viewed the entire history of life, the full tree itself, as growing
by an adaptive and stately unfolding, then the history of single branches —trends
in lineages, the primary topic of macroevolutionary study—also received a
thoroughly adaptationist reading in the extrapolationist mode. At the same Chicago
conference, Simpson defended the adaptationist postulate for all geometries,
parallel as well as diverging lineages, and even (in principle and without direct
evidence) for "erratic" features where selective control "is not apparent."
The selectionist theory is that a trend is adaptive for the lineage involved,
that it continues only as long as it is adaptive, that it stops when adaptation
is as complete as selection can make it in given circumstances, and that it
changes or the group becomes extinct if a different direction of evolution
becomes adaptive. Often the adaptive nature of a trend seems apparent.
Often it is not apparent, but the postulate seems required to account for
otherwise erratic features of trends. In instances of parallel evolution the
selectionist theory is that changes actually occurring in parallel are adaptive
over the whole ecological range occupied by the group, while those
divergent (radiating) within the group are adaptations to special niches
within that range (Simpson, 1960, pp. 170-171).Under these precepts, a procedure of building scenarios in the strictly
adaptationist mode, based on assumption and conjecture, often passed for adequate
explanation. The second half of the Chicago panel on the "evolution of life,"
supposedly dedicated to the actual record of evolutionary change, included almost
no discussion about paleontology, and relied on theoretical inferences about the
past based on knowledge of modern organisms. Ernst