What is punctuated equilibrium?

Evolutionary Patterns

Nineteenth century English naturalist Charles Darwin
viewed the development of new species as occurring slowly by a shift of characteristics within populations, so that a gradual transition from one species to another took place. This is now generally referred to as phyletic gradualism. A number of examples from the fossil record
were put forward to support this view, particularly that of the horse, in which changes to the feet, jaws, and teeth seem to have progressed in one direction over a long period of time. Peter Sheldon in 1987 documented gradual change in eight lineages of trilobites over a three-million-year period in the Ordovician period of Wales. Despite these and other examples (some of which have been reinterpreted), it is clear that the fossil record more commonly shows a picture of populations that are stable through time but are separated by abrupt morphological breaks. This pattern was recognized by Darwin but was attributed by him to the sketchy and incomplete nature of the fossil record. So few animals become fossilized, and conditions for fossilization are so rare, that he felt only a fragmentary sampling of gradual transitions was present, giving the appearance of abrupt change.

One hundred years later, the incompleteness of the fossil record no longer seemed convincing as an explanation. In 1972, Niles Eldredge and Stephen Jay Gould
published their theory of the evolutionary process, which they called “punctuated equilibrium.” This model explains the lack of intermediates by suggesting that evolutionary change occurs only in short-lived bursts in which a new species arises abruptly from a parent species, often with relatively large morphological changes, and thereafter remains more or less stable until its extinction.

The Process of Punctuated Equilibrium

A number of explanations have been put forward to show how this process might take place. One of these, termed allopatric speciation,
was first proposed by Ernst Mayr in 1963. He pointed out that a reproductive isolating mechanism is needed to provide a barrier to gene flow and that this could be provided by geographic isolation. Allopatric or geographical isolation could result when the normal range of a population of organisms is reduced or fragmented. Parts of the population become separated in peripheral isolates, and if the population is small, it may become modified rapidly by natural selection or genetic drift, particularly if it is adapting to a new environment. This type of process is commonly called the founder effect, because it is the
characteristics of the small group of individuals that will overwhelmingly determine the possible characteristics of their descendants. As the initial members of the peripheral isolate may be few in number, it might take only a few generations for the population to have changed enough to become reproductively isolated from the parent population. In the fossil record, this will be seen as a period of stasis representing the parent population, followed by a rapid morphological change as the peripheral population is isolated from it and then replaces it, either competitively or because it has become extinct or has moved to follow a shifting habitat. Because this is thought to take place rapidly in small populations, fossilization potential is low, and unequivocal examples are not common in the fossil record. However, in 1981, Peter Williamson published a well-documented example from the Tertiary period of Lake Turkana in Kenya, which showed episodes of stasis and rapid change in populations of freshwater
mollusks. The increases in evolutionary rate were apparently driven by severe environmental change that caused parts of the lake to dry up.

Punctuated changes may also have taken place because of heterochrony, which is a change in the rate of development or timing of appearance of ancestral characters. Paedomorphosis, for example, would result in the retention of juvenile characters in the adult, while its opposite, peramorphosis, would result in an adult morphologically more advanced than its ancestor. Rates of development could be affected by a mutation, perhaps resulting in the descendant growing for much longer than the ancestral form, thus producing a giant version. These changes would be essentially instantaneous and thus would show as abrupt changes of species in the fossil record.

Impact and Applications

The publication of the idea of punctuated equilibrium ignited a storm of controversy that still persists. It predicts that speciation can be very rapid, but more important, it is consistent with the prevalence of stasis over long periods of time so often observed in the fossil record. Species had long been viewed as flexible and responsive to the environment, but fossil species showed no change over long periods despite a changing environment. Biologists have thus had to review their ideas about the concept of species and the processes that operate on them. Species are now seen as real entities that have characteristics that are more than the sum of their component populations. Thus the tendency of a group to evolve rapidly or slowly may be intrinsic to the group as a whole and not dependent on the individuals that compose it. This debate has helped show that the fossil record can be important in detecting phenomena that are too large in scale for biologists to observe.

Key terms



allopatric speciation

:

a theory that suggests that small parts of a population may become genetically isolated and develop differences that would lead to the development of a new species





heterochrony


:

a change in the timing or rate of development of characters in an organism relative to those same events in its evolutionary ancestors




phyletic gradualism

:

the idea that evolutionary change proceeds by a progression of tiny changes, adding up to produce new species over immense periods of time





Bibliography

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Eldredge, Niles, and Stephen Jay Gould. “Punctuated Equilibria: An Alternative to Phyletic Gradualism.” Models in Paleobiology. Ed. Thomas J. M. Schopf. San Francisco: Freeman, 1972. Print.

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