Evolution: Tempo and Mode


‘Tempo’ refers to the rate of evolution, the rate of phenotypic change either within lineages or expressed as averages within major taxonomic groups. ‘Mode’ refers to the pattern of evolution – whether it is continuous or episodic; whether it is primarily phyletic (within‐lineage) or proceeds mostly by lineage‐splitting; whether it shows directional trends or not; and whether it proceeds by small steps or sudden jumps.

Keywords: evolutionary rates; punctuated equilibrium; gradualism; speciation; stasis

Figure 1.

Simpson's three primary modes of evolution. The bell‐shaped curves are meant to suggest the distribution of morphology at any given time, and the dashed lines the ancestor–descendant pathways. The solid parallel lines indicate ‘adaptive zones’. (a) Speciation: population differentiation, race formation, and formation of species without significant change in ecological niche. (b) Phyletic evolution: the transformation of whole lineages by slow and progressive evolution – phyletic gradualism is shown, but Simpson also included stasis under this heading (see text). (c) Quantum evolution: the rapid transition from one adaptive zone to another. Similar to punctuated equilibrium, except that for Simpson this occurred only in cases where major niche shifts were involved. After Simpson (1944), figure 31.

Figure 2.

Schematic representation of evolutionary patterns in lower molar tooth area in species of the small Eocene mammal, Hyopsodus, from northwestern Wyoming. A total of three species lineages are shown (lineages represented by only one sample have been omitted). Shaded regions indicate intervals of intensive sampling at 6 m intervals in the section of sediments that spans a total of 500 m. Width of the lineages indicates the observed range of variation in morphology, and the thin dashed lines track the means of samples from successive levels. The thick horizontal dashed line in the centre of the section represents a species transition in a poorly sampled interval. After Gingerich PD (1977) Patterns of evolution in the mammalian fossil record. In: Hallam A (ed.) Patterns of Evolution, pp. 469–500. Elsevier, Amsterdam.

Figure 3.

Punctuated equilibrium and phyletic gradualism. The evolution of six lineages involving approximately the same range and sequence of morphological change, according to the two theories. Under punctuated equilibrium (a), the species are in morphological stasis throughout their durations, and the rise and establishment of new, descendant species (dashed lines) is so rapid as to be unobserved. Under phyletic gradualism (b), evolutionary change is happening constantly in all lineages, at varying (but usually slow) rates, and new species are formed by gradual transitions, at rates similar to those observed within species lineages.


Further Reading

Bookstein FL (1987) Random walk and the existence of evolutionary rates. Paleobiology 13: 446–464.

Brett CE, Ivany LC and Schopf KM (1996) Coordinated stasis: an overview. Palaeogeography, Palaeoclimatology, Palaeoecology 127: 1–20.

Charlesworth B, Lande R and Slatkin M (1982) A Neo‐Darwinian commentary on macroevolution. Evolution 36: 474–498.

Eldredge N and Gould SJ (1972) Punctuated equilibria: an alternative to phyletic gradualism. In: Schopf TJM (ed.) Models in Paleobiology, pp. 82–115. San Francisco: Freeman, Cooper.

Fitch WM and Ayala FJ (eds) (1995) Tempo and Mode in Evolution: Genetics and Palaeontology 50 Years after Simpson. Washington, DC: National Academy Press.

Gillespie JH (1991) The Causes of Molecular Evolution. New York: Oxford University Press.

Gingerich PD (1993) Quantification and comparison of evolutionary rates. American Journal of Science 293‐A: 453–478.

Gould SJ and Eldredge N (1977) Punctuated equilibria: the tempo and mode of evolution reconsidered. Paleobiology 3: 115–151.

Gould SJ and Eldredge N (1993) Punctuated equilibrium comes of age. Nature 366: 223–227.

Lande R (1976) Natural selection and random genetic drift in phenotypic evolution. Evolution 30: 314–334.

Lees DR and Edwards D (eds) (1993) Evolutionary Patterns and Processes. Linnaean Society Symposium Series, vol. 14. London: Academic Press.

Martin RA and Barnosky AD (eds) (1993) Morphological Change in Quaternary Mammals of North America. New York: Cambridge University Press.

Schopf JW (1995) Disparate rates, differing fates: tempo and mode of evolution changed from the Precambrian to the Phanerozoic. In: Fitch WM and Ayala FJ (eds) Tempo and Mode in Evolution: Genetics and Palaeontology 50 Years after Simpson, pp. 41–61. Washington, DC: National Academy Press.

Simpson GG (1944) Tempo and Mode in Evolution. New York: Columbia University Press.

Stanley SM (1979) Macroevolution: Pattern and Process. San Francisco: WH Freeman.

Stanley SM and Eldredge N (eds) (1984) Living Fossils. New York: Springer Verlag.

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Damuth, John D(Apr 2001) Evolution: Tempo and Mode. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1038/npg.els.0001720]