Punctuated equilibrium
Punctuated equilibrium is a model of evolutionary change that posits new species originate in relatively rapid bursts, followed by long periods of stability, or stasis. This concept contrasts with the traditional view of phyletic gradualism, where species evolve slowly and continuously over time. Introduced by paleontologists Niles Eldredge and Stephen Jay Gould in 1972, punctuated equilibrium emerged as a response to patterns observed in the fossil record that frequently show species remaining unchanged for extended periods, interspersed with sudden appearances of new forms.
The theory suggests that rapid changes can occur during specific events, such as geographic isolation of populations, which allows for intense natural selection or genetic drift, leading to significant morphological alterations. This is known as allopatric speciation, where small, isolated groups evolve into distinct species. The model has prompted a reevaluation of how biologists understand species dynamics, emphasizing that the evolutionary pace may be intrinsic to certain groups rather than solely a response to environmental pressures. Punctuated equilibrium continues to provoke discussion and research in evolutionary biology, highlighting the complexities of how species adapt and change over time.
Punctuated equilibrium
SIGNIFICANCE: Punctuated equilibrium is a model of evolutionary change in which new species originate abruptly and then exist through a long period of stasis. This model is important as an explanation of the stepwise pattern of species change seen in the fossil record.
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 in Wales.
Despite these and other examples (some of which have since been reinterpreted), it is clear that the fossil record more commonly shows a picture of populations that are largely stable through time, 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. Mayr 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 the characteristics of a small group of individuals 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 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 speciationa 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
- heterochronya change in the timing or rate of development of characters in an organism relative to those same events in its evolutionary ancestors
- phyletic gradualismthe idea that evolutionary change proceeds by a progression of tiny changes, adding up to produce new species over immense periods of time
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