Definition
Punctuated equilibrium is a theory that states that evolution occurs primarily through short bursts of intense speciation, followed by lengthy periods of stasis or equilibrium. The model postulates that nearly 99% of a species’ time on earth is spent in stasis, and change happens very quickly.
Punctuated Equilibrium Explained
So, if a species appears in fossil records for about 10 million years, it is likely that speciation occurred over the span of fewer than 100,000 years. Once complete, however, the model suggests that there is little morphological change after the speciation event. The species will then maintain a period of stability, called stasis, for a long period of time.
The theory also provides a reasonable explanation for the absence of intermediate forms in fossil records, where new species seem to appear from ancestral forms abruptly and ultimately disappear without experiencing any apparent morphological change during their existence.
While this was a shift from the idea that all new species arose due to continuous, gradual and incremental changes, the founders of this theory have also conceded that other modes of evolution could co-exist. The opposite theory is gradualism, which suggests that change occurs consistently, over long periods of time.
Punctuated Equilibrium Examples
Support for punctuated equilibrium is seen in fossil records. The impact of reproductive isolation has been observed by biologists, systematists, and taxonomists across the world. Given the fact that this is a theory of evolution, its predictions cannot be directly tested.
While the fossil record can provide support for the theory, some indications need to arise from the living world. For example, animals living in similar environments which experience reproductive isolation must become incapable of interbreeding, indicating the emergence of a new species. This has been observed several times.
Reproductive Isolation among Kingfishers
The study of kingfishers in Papua New Guinea showed the deep impact of reproductive isolation on speciation. There are three subspecies that reside on the mainland. Here, the environment can vary wildly from humid, dense rain forest to monsoon forests with extended dry seasons. These subspecies can not only interbreed but are nearly indistinguishable from each other.
However, on islands a few hundred kilometers away, even when the environment is similar to the nearest part of the mainland, the kingfishers are markedly different. More species have been found on these smaller islands than on the mainland. Similar observations have been made for birds and reptiles and invertebrates across the world, where geographical separation has led to the emergence of new species, while large continuous tracts with varying conditions maintain homogeneous populations. This evidence supports the punctuated equilibrium theory.
Land Snails of Bermuda
About 300,000 years ago, Poecilozonites bermudensis, an air-breathing land snail, colonized the island of Bermuda. The snail might have been carried on driftwood from North America. The fossils of these snails constitute the large majority of Bermuda’s land fossils.
Until recently, one species continued to survive on the island. The earliest populations of this snail had two stocks, with distinct color banding patterns. When these became extinct, a derivative from a peripheral population that was evolving on a separate island became dominant. Fossil samples taken from six different geological times and from various geographical locations points to the repeated evolution of species from peripherally isolated populations that ultimately led to the formation of the land snail that remained morphologically static until it was observed in the 1950s.
Features of Punctuated Equilibrium
One of the cornerstones of this hypothesis is that reproductive isolation is necessary for the formation of new species. This implies that the fossil record at any one place is unlikely to record the process of speciation because new species can evolve only from small, isolated populations. Therefore, variations will be seen only in fossils of the same age arising from different geographical locations.
Rapid Morphological Changes
Punctuated equilibrium postulates that genetic and morphological changes that bestow a survival advantage will be amplified quickly in small populations. The rapid pace of evolution in these isolated groups is also stated as the reason why there is no fossil record of evolution, and new species seem to appear abruptly.
It also predicts that while intermediates will be rare in the evolution of single species, they will be seen among larger groups. For example, while, Australopithecus afarensis, is the precursor of modern humans, there are no fossils showing a gradual change in the cranial capacity or body size of the Australopithecus. However, there are other species such as Homo habilis and Homo erectus that show the transition from Australopithecus towards a modern man in terms of cranial capacity and body size.
Periods of Stasis
Another important feature of this hypothesis is the explanation given for extended periods of stasis. It implies that the average morphology of a species is under a homogenizing influence. Interbreeding populations appear static. This is due to the fact that small changes within a large population become diluted and homogenized.
A number of explanations have been given for this phenomenon observed in the fossil record. These include the effect of gene flow, assertions that the morphology of a species is under ‘homeostatic’ pressure, and koinophilia or the rejection of mates with unusual attributes.
Gradualism vs Punctuated Equilibrium
Punctuated equilibrium is pitted against phyletic gradualism. These competing theories of evolution both hypothesize about the rate of emergence of new species. Gradualism places importance on the slow appearance of new characters in interbreeding subspecies that, over time, lead to the evolution of a new species from ancestral forms.
Fossils seem to appear suddenly throughout history. Punctuated equilibrium tries to explain these fossil ‘gaps’ or the absence of intermediate forms, by stating that they exist for very short periods of time when speciation occurs intensely in an isolated population.
The criticism of punctuated equilibrium focuses on the possibility that fossil records may simply be incomplete. Intermediate forms may be found in regions where fossils are abundant and well-preserved. In addition, critics point to the fact that there is no evidence that an external homogenizing influence keeps interbreeding populations in stasis.
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