Punctuated Equilibrium v. Phyletic Gradualism

Punctuated Equilibrium v. Phyletic Gradualism One of the biggest debates in the theory of evolution over the past thirty years has been between proponents of phyletic gradualism, who believe that evolution takes place at a fairly even -- though not necessarily smooth -- pace, and those who belong to the school of punctuated equilibrium, whose view is that evolution occurs in fits and starts, with long periods of stasis between evolutionary leaps.

Both of these theories use the fossil record as evidence that suggests they are correct, and it is possible that the truth behind evolution contains elements of both phyletic gradualism and punctuated equilibrium. Because of the incompleteness of the fossil record, however, the truth about evolution will probably never be known for certain. There is some disagreement on whether or not Darwin believed in phyletic gradualism.

Stephen Jay Gould, one of the two scientists who first theorized the process of punctuated equilibrium in 1972, claims that the theory actually fits with most of Darwin's original theory of evolution, and only "refutes the third and most general meaning of Darwinian gradualism...[a] 'slowness and smoothness (but not constancy) of rate'" (Gould, 27). Gould argues that punctuated equilibrium does not deny the operation of natural selection, which many paleontologists and other scientists take as evidence of gradualism because of the small changes in individuals it causes.

Gould claims that this is an incorrect interpretation of natural selection, claiming that "natural selection does not require or imply this degree of geological sloth and smoothness" (Gould, 27). Instead, according to the theory of punctuated equilibrium, natural selection occurs at population fringes, or when populations become separated (Ridley, 599). Small changes in small populations can accumulate rapidly, and cause speciation in a punctuated way, perhaps only taking a few generations to produce a non-interbreeding species separate from its ancestral group.

Until the theory of punctuated equilibrium was postulated, phyletic gradualism was accepted from Darwin's time on as the most likely mechanism by which macroevolution -- large-scale evolution dealing with a population rather than an individual lineage -- occurs. It was believed by Darwin and many others after him that the process of natural selection would cause a slow and gradual change in a population of species, resulting in the formation of new species over long periods of time, with little discernible difference from generation to generation (Ridley, 594).

This theory also suggests, in contrast to punctuated equilibrium, that changes generally occur in populations as a whole, rather than merely on the fringes as Gould and others suggest (KVIE, par. 4). The main problem with this theory is that the fossil record does not contain evidence of many intermediary species -- that is, the series species with minute changes that ought to exist between obviously related species with major changes. The fossil record is the main source of evidence and consternation for both theories.

Evidence, because it is the only way scientists can look into the distant biological past and so come up with ling-term explanations for macroevolution, and consternation because its incompleteness leaves it open to a diversity of equally plausible explanations. Gould and other punctuated equilibrium proponents see the lack of intermediary species as a flaw in the theory of phyletic gradualism; they see this as convincing evidence that species make bigger evolutionary leaps in shorter time frames than gradualism allows for.

Gradualists actually take this lack of fossil evidence as possible proof that their theory is correct -- the incompleteness of the fossil record makes it far less likely to contain record of transitional species (KVIE, pars. 2-3). The theory of punctuated equilibrium, with its reliance on the facts present rather than an insistence on facts absent, makes more sense to my mind than the theory of phyletic gradualism.

Gould's explanations extend beyond the mere mechanism of macroevolution to explain -- not with certainty, but with great plausibility -- the state of the fossil record. If, as Gould proposes, small populations of a species changed, then "the new species will only leave fossils at the same site as the old one if it becomes successful enough to move back into its ancestral range or different enough to exist alongside its relatives" (KVIE, par. 5).

This is an actual explanation for the fossil record, rather than an insistence on its incompleteness as reason not to discount the theory of phyletic gradualism. Having a better explanation for the fossil record in no way proves the theory of punctuated equilibrium, any more than it disproves phyletic gradualism, but its reliance on known facts makes it the most.