Punctuated Equilibrium

Punctuated equilibrium is a theory about how new species evolve that was first advanced by American paleontologists Niles Eldredge and Stephen Jay Gould (1941–2002) in 1972. Although controversial, punctuated equilibrium has stimulated fruitful debate about speciation (the birth of new species) and the fossil record and has, in recent years, won at least partial acceptance among most evolutionary biologists.

Before punctuated equilibrium, most scientists assumed that evolutionary change occurs slowly and continuously in almost all species, and that new species originate either by slow divergence from parental stock of sub-populations or by slow evolutionary transformation of the parental stock itself. Punctuated equilibrium proposes that most species originate relatively suddenly (i.e., over tens of thousands or hundreds of thousands of years, rather than the millions of years assumed by traditional theory) and then do not evolve significantly for the rest of their time on Earth. Most species thus have a sudden or punctuated origin and then remain in stasis or equilibrium until extinction.

Eldredge and Gould proposed punctuated equilibrium to explain one of the most notable features of the fossil record: most species seem to appear suddenly, already clearly differentiated from the earlier, similar species from which they presumably evolved, and then remain unchanged until becoming extinct. (Most species become extinct a few million years after appearing; a few last for tens of millions of years or longer.) Traditional evolutionary theory, beginning with the Origin of Species (1859) by English naturalist Charles Darwin (1809–1882), proposed that gradual evolutionary changes are rarely observed in the fossil record because that record is radically incomplete. Fossils form only under certain special conditions, fossil-bearing rocks are eroded as well as deposited, and our knowledge even of those fossils that have been formed is fragmentary. It follows that in the fossil record we glimpse only a few isolated frames cut at long intervals from a long, slow-moving film. As Darwin himself put it, "I look at the natural geological record, as a history of the world imperfectly kept ... of this history we possess the last volume alone, relating only to two or three countries. Of this volume, only here and there a short chapter has been preserved; and of each page, only here and there a few lines."

Eldredge and Gould agree that the fossil record is incomplete, but contend that it could not be incomplete enough to account for the near-complete absence of gradualistic change from the fossil record. Rather, they propose, species normally originate too quickly for normal geological processes to record the event; a single bedding plane (minimal layer of sedimentary rock) often compresses tens of thousands of years into a thin slice. Furthermore, according to standard evolutionary theory of 1972, speciation usually occurs when small populations cut off from interbreeding with related gropus—say, by loss of a watercourse connecting two lakes, or by colonization of an island—evolve rapidly in isolation. Because there are fewer individuals in such an isolated population, favorable mutations can spread more readily. A small, isolated, rapidly-evolving population may become extinct without leaving any trace at all in the fossil record. Eldredge and Gould argued that if it does eventually break out of its isolation and spread over a wider area, it is likely to be observed in the fossil record as making a sudden or punctuational appearance, fully formed. They also proposed that the appearance of stasis or unchanging form manifested by most species in the fossil record is not an artifact produced by gross imperfections in the fossil record, but a raw fact. Evolutionary change in living forms, the two scientists argued, occurs mostly during speciation events and hardly otherwise.

It is important to note that by the standards of recorded human history, which covers only about 7,000 years, speciation is still a very gradual process under punctuated equilibrium theory. Punctuated equilibrium argues for much faster speciation than traditional evolutionary theory, but does not involve the proposition that new species appear in a generation or two. It is an evolutionary theory according to which hundreds or thousands of generations are needed for speciation, and natural selection must favor (or at least permit) all changes at every step. The novelty of punctuated equilibrium lies in its two proposals about rates of evolutionary change: (1) change happens rapidly, by geological standards, during speciation, and (2) change happens slowly or not at all after speciation.

A growing body of evidence indicates that both gradualistic and punctuational speciation have often occurred in the history of life, and that morphological stasis (long-term stability of form)—the "equilibrium" of "punctuated equilibrium"—is, as Eldredge and Gould claimed, often real, rather than an artifact of dropout in the fossil record. Several unusually perfect series of fossils have been discovered that have allowed paleontologists (fossil specialists) to trace the detailed history of entire groups of related organisms. In most such cases, paleontologists have observed gradualistic speciation, punctuational speciation, and morphological stasis, all in a single series of rocks, with punctuational speciation occurring about 10 times more frequently than gradualistic speciation. Observing gradualistic speciation and punctuational speciation in a single series of fossils proves both gradualism by direct observation, and punctuated equilibrium by disproof of the alternative possibility that gaps are responsible for the relatively sudden appearance of species in this case.

Evolutionary biologists continue to debate the question of relative frequency, that is, which happens more frequently in the history of life: gradualistic evolution or punctuated equilibrium? Although scientists who support punctuated equilibrium claim that the evidence shows a much greater relative frequency for punctuated equilibrium, debate continues.