Punctuated Equilibrium
Punctuated equilibrium, also called punctuated equilibria or the punctuational model, holds that genetic change occurs relatively rapidly on a geological timescale, and these shorter periods of evolution are sandwiched between longer periods of “stasis,”1 during which little or no net change occurs. This view of evolutionary change stands in contrast to Darwin’s traditional view of gradualism, where evolution proceeds at a slow, continuous pace (often called the “gradualistic model”).
Darwin attributed the abrupt, transitionless appearance of novel forms in the fossil record to the imperfection of the record.2 The punctuated equilibria model was first proposed by paleontologists Niles Eldredge and Stephen Jay Gould in the early 1970’s. Eldredge suggested that when that the fossil record told a story that species had appeared abruptly, that the record was not entirely imperfect and the pattern of abrupt appearance could be taken at face value.3 Douglas Futuyma thus characterizes their model as follows:
“The hypothesis of punctuated equilibrium holds (among other things), that most evolutionary changes in morphology, although perhaps continuous in the sense of passing through many intermediate stages, have been so rapid that the fossil record presents the appearance of a discontinuous change.”4
Proponents of the punctuational model have relied heavily upon the work of mathematician Sewall Wright, who observed that novel variation was more likely to become fixed in small populations which were reproductively isolated (often due to geographic factors) from the main population.5 Yet in larger populations, the odds of a given trait becoming fixed, especially in a relatively short period of time, are much smaller. The achievement of reproductive isolation, often called speciation, can thus foster the promulgation of novel variation (called “fixation”) into a population . This genetically distinct population might be thus labeled a new species.
According to Gould and Eldredge, speciation events are still “gradual,” in the sense that evolution must still proceed by natural selection acting upon very small-steps of intermediate variation in the speciating population.6 The punctuational model should thus not be confused with a saltational model of evolutionary change, where intermediate forms never even existed.7
Various estimates have been put forth for how long a speciation event might take, but statements by Gould and Eldredge have ranged from anywhere from estimating hundreds, to 50,000 years.8 According to Gould and Eldredge, such a small amount of geological time would represent only a thin slice of bedding plane in rock strata, and thus the chances that an organism bearing intermediate, transitional morphology might get fossilized is greatly diminished.9
The punctuational model states that species do not arise as an entire population evolves, but rather when a segment of a population becomes isolated and undergoes a speciation event. One primary difference between the punctuational and gradualistic models of evolution is thus that under the gradualistic model, the entire parent population slowly morphs into the descendant species. However, under the punctuational model, only a small segment of the parent population “buds” off into the descendant population, permitting the original parent population to continue to persist. The punctuational model may thus be validated if a new species emerges but the parent species remains intact.10 Details of the nature of speciation events are difficult to eludicate because they are too slow to observe on the timescale of human studies, but too rapid to be recorded by the fossil record.11
Punctuated equilibrium has seen both strong supporters and detractors in the evolutionary biology community. Gould himself predicted that “[p]unctuated equilibrium will be validated, as all such theories in natural history must be (including natural selection itself), by predominant relative frequency, not by exclusivity.”12
Notes
(1) “Stasis is not defined as absolute phenotypic immobility, but as fluctuation of means through time at a magnitude not statistically broader than the range of geographic variation among modern populations of similar species, and not directional in any preferred way, especially not towards the phenotype of descendants.” Stephen Jay Gould, The Structure of Evolutionary Theory, pg. 76 (Harvard University Press 2002).
(2) Gould, The Structure of Evolutionary Theory, pgs. 75, 750.
(3) Niles Eldredge and Ian Tattersall, The Myths of Human Evolution, pg. 59 (Columbia University Press, 1982).
(4) Douglas Futuyma, Evolutionary Biology, pg. 677 (Sinauer Associates, 1998).
(5) Steven Stanley, The New Evolutionary Timetable, pg 69 (Basic Books, 1981).
(6) Futuyma, Evolutionary Biology, pg. 139.
(7) Futuyma, Evolutionary Biology, pgs. 677-678.
(8) Stephen Jay Gould, quoted in Teaching About Evolution and the Nature of Science, pg. 56 (National Academy Press, 1998).
(9) Stephen Jay Gould, “Is a new and general theory of evolution emerging?” Paleobiology, 6(1):119-130 (1980).
(10) Gould, The Structure of Evolutionary Theory, pg. 76.
(11) Futuyma, Evolutionary Biology, pg. 447.
(12) Gould, The Structure of Evolutionary Theory, pg. 76.
Web Resources On Punctuated Equilibrium
Opus 200 by Stephen Jay Gould
Punctuated Equilibria by Wesley Elsberry
Punctuated Equilibrium and Patterns from the Fossil Record
Book Resources On Punctuated EquilibriumThe Structure of Evolutionary Theory by Stephen Jay Gould
The New Evolutionary Timetable by Steven Stanley
Reinventing Darwin by Niles Eldredge
Editor(s): Casey Luskin
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