Tempo and Mode in Evolution (1944) was George
Gaylord Simpson's seminal contribution to
the evolutionary synthesis, which integrated
the facts of paleontology with those of genetics
and natural selection.
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).
The facts of paleontology are consistent with
the genetical theory of natural selection.
Moreover, theories such as orthogenesis, Lamarckism,
mutation pressures, and macromutations either
are false or play little to no role.
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.Tempo and Mode
earned Simpson the Daniel Giraud Elliot Medal
from the National Academy of Sciences in 1944.
Fifty years after its publication, the National
Academy of Sciences commissioned a book entitled
Tempo and Mode in Evolution: Genetics and
Paleontology 50 Years After Simpson edited
by Walter M. Fitch and Francisco J. Ayala.
It includes contributions by Ayala, Stephen
Jay Gould, and W. Ford Doolittle
