562 THE STRUCTURE OF EVOLUTIONARY THEORY
as a device to provide more opportunities for adaptation to work its "untrammeled"
magic: "The splitting of organisms into the genetically isolated groups we call
'species' has played a very important part in evolution, for it has permitted selection
to proceed untrammeled within each group, so permitting adaptations of
innumerable kinds in the different groups. Had organisms not divided into
genetically isolated groups, the numerous and beautiful adaptations so
characteristic of living things could not have evolved, nor could organisms have
used the resources of the world in the efficient way they do" (Nicholson, 1960, p.
518).
More commonly, however, speciation received short shrift rather than glory.
Evolution required such a process of multiplication, of course, lest favorable trends
disappear through the extinction of single species bearing their fruits. Speciation
therefore became a hedge against death by parcelling out, into several iterated
lines, a set of adaptations built anagenetically—so that the extinction of one
species could not abort the general benefit. The trend itself remains anagenetic (see
Fig. 7-2), for speciation does not contribute to the directionality of evolutionary
change. (Under later views, including punctuated equilibrium, differential
speciation constructs the trend, and anagenetic main trunks do not even exist.)
Simpson held strongly to this view, and even ventured a quantitative defense,
in his repeated assertions that speciation represents only a minor mode in evolution
because 90 percent of important changes arise anagenetically in the phyletic mode
(1944, 1953; Simpson recognized three major modes of change: speciation,
phyletic evolution, and quantum evolution). Huxley, in a grand prose flourish, then
branded speciation as a pretty little epiphenomenon, a luxurious patina upon the
grand pattern of evolution—never realizing
7 - 2. Standard view of the role of speciation in evolutionary trends under the Modern Synthesis.
Speciation certainly plays an important part in iterating favorable variations produced by
anagenesis within species. (If this iteration did not occur, lineages would quickly become extinct
because individual species must eventually die.) But the trend in morphology arises almost
entirely by anagenetic directionalism within the geological duration of individual species.