Simpson argued that the
microevolution of
population genetics was sufficient in itself to explain the patterns of
macroevolution observed by
paleontology. Simpson also highlighted the distinction between tempo and mode. "Tempo" encompasses "evolutionary rates … their acceleration and deceleration, the conditions of exceptionally slow or rapid evolutions, and phenomena suggestive of inertia and momentum," while "mode" embraces "the study of the way, manner, or pattern of evolution, a study in which tempo is a basic factor, but which embraces considerably more than tempo."
Simpson's Tempo and Mode attempted to draw out several distinct generalizations:
Evolution's tempo can impart information about its mode.
Multiple tempos can be found in the fossil record: horotelic (medium tempo), bradytelic (slow tempo), and tachytelic (rapid tempo).
Most evolution—"nine-tenths"—occurs by the steady phyletic transformation of whole lineages (
anagenesis). This contrasts with
Ernst Mayr's interpretation of speciation by splitting, particularly
allopatric and
peripatric speciation.
The lack of evidence for evolutionary transitions in the fossil record is best accounted for, first, by the poorness of the geological record, and, second, as a consequence of
quantum evolution (which is responsible for "the origin of taxonomic units of relatively high rank, such as families, orders, and classes"). Quantum evolution built upon
Sewall Wright's theory of
random genetic drift.
Gould, S. J. (1980) "G. G. Simpson, Paleontology and the Modern Synthesis." In
E. Mayr and
W. B. Provine, eds., The Evolutionary Synthesis. Cambridge MA: Harvard University Press, pp. 153–172.
Gould, S. J. (1983) "The hardening of the Modern Synthesis". In Marjorie Grene, ed., Dimensions of Darwinism. Cambridge UK: Cambridge University Press, pp. 71–93.