Punctuated Equilibrium Scientists attempt to explain the world

Punctuated Equilibrium

• Scientists attempt to explain the world around us. • They do this through a series of proposed explanations for observed phenomena called hypotheses. • These hypotheses are then tested over a period of years to determine if they are supported by facts or if they need to be revised.

• Take, for example, evolution. • Scientists have long studied how new species come to be in the world and how these new species descend from ancestors. • The exact mechanism that drives this speciation still eludes (escape, avoid) scientists to some extent, so they are constantly attempting to come up with hypotheses to try and explain their observations.

• One major observation about the world around us has to do with when new species appear in the fossil record. • Has the evolution of life proceeded as a gradual stepwise process, or through relatively long periods of stasis punctuated by short periods of rapid evolution?

• To date, what is clear is that both evolutionary patterns – phyletic gradualism and punctuated equilibrium have played at least some role in the evolution of life. • Gradualism and punctuated equilibrium are two ways in which the evolution of a species can occur. • A species can evolve by only one of these, or by both.

• Both phyletic gradualism and punctuated equilibrium are speciation theory and are valid models for understanding macroevolution. • Phyletic gradualism was proposed by Charles Darwin • However, the punctuated equilibrium was proposed as an alternative hypothesis by Eldridge and Gould to explain the gaps in the fossil record

• The gradualism model depicts evolution as a slow steady process in which organisms change and develop slowly over time. • In contrast, the punctuated equilibrium model depicts evolution as long periods of no evolutionary change followed by rapid periods of change. • Both are models for describing successive evolutionary changes due to the mechanisms of evolution in a time frame.

• Scientists think that species with a shorter evolution evolved mostly by punctuated equilibrium, and those with a longer evolution evolved mostly by gradualism. • Punctuated equilibrium and phyletic gradualism are contrasting patterns of evolution among a spectrum of patterns found in the fossil record. • In punctuated equilibrium, species tend to show morphological stasis between abrupt speciation events, whereas in phyletic gradualism species undergo more continuous change

• Punctuated equilibrium • (Punctuated: interrupted, disrupted, scattered; equilibrium: balance, symmetry, stability) • is a hypothesis devised to explain a pattern of change in the fossil record and how closely related species appear in nature. • The hypothesis states that individual species tend to show little or no change over a long period of geological time and then enter a period of rapid change, which gives rise to new species.

• Further it states that evolution occurs primarily through short bursts of intense speciation, followed by lengthy periods of stasis or equilibrium. • It postulates that nearly 99% of a species’ existence on earth is spent in stasis. • So, if a species appears in fossil records for about 10 million years, it is likely that speciation occurred over the span of less than 100, 000 years. • Once complete however, there is little, if any, morphological change.

• The theory also provides an 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.

Features of Punctuated Equilibrium • The essential features that make up Punctuated Equilibrium are as follows: • Most speciation is cladogenesis rather than anagenesis; • Most speciation occurs through peripatric speciation; • Large, widespread species don’t change or change slowly; • Daughter species develop in a geographically limited region;

(a) Anagenesis is the change of one species into another, whereas (b) cladogenesis occurs when one population diverges into two or more different species.

• Daughter species develop stratigraphically limited extent; in a • Sampling of the fossil record will reveal a pattern of most species in stasis, with abrupt appearance of new species; • Adaptive change in lineages occurs mostly during periods of speciation; • Trends in adaptation occur through the mechanism of species selection.

• Furthermore, Punctuated Equilibrium holds that: • Evolutionary change is connected in speciation events. • Most species remain pretty much the same once they have come into being. • This lack of substantial change over millions of years is called stasis.

• Speciation events are normally confined to small populations peripheral isolates that have become separated from the bulk of the species. • As these isolated populations are small and transient, we should not expect to find them in the fossil record. • So the absence of the missing links is neatly explained by this theory.

• One of the cornerstones (keystone, basis, foundation) 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.

• 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 modern man in terms of cranial capacity and body size.

• 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 are said to appear static because, in the absence of active selection pressure, any changes are diluted among the large number of individuals.

• A number of explanations have been given for this phenomenon observed in 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.

• Examples of Punctuated Equilibria • Support for punctuated equilibrium is seen in fossil records, and 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 fossil record can provide support for theory, some indications need to arise from the living world. • For example, animals living in similar environments but experiencing reproductive isolation must become incapable of interbreeding, indicating the emergence of a new species.

• 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, where 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 in mainland – a landmass spanning nearly 300, 000 square miles. • 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.

• Land Snails of Bermuda • About 3 lac years ago, Poecilozonites bermudensis, an air-breathing land snail, colonized the island of Bermuda, possibly carried on driftwood from North America. • The fossils of these snails constitute the large majority of Bermuda’s land fossils and, 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 till it was observed in the 1950 s.

• Gradualism vs Punctuated Equilibrium • Punctuated equilibrium is often pitted against phyletic gradualism as competing theories of evolution. • Both of these hypothesize about the rate of emergence of 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.

• However, this is not supported by fossil data, where new species seem to appear suddenly. • Punctuated equilibrium tries to explain these fossil ‘gaps’ or the absence of intermediate forms, but 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 and intermediate forms may still 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.

• Allopatric speciation – Speciation that occurs after a population splits into two groups that are reproductively isolated from each other. • Koinophilia – Phenomenon where individuals with unusual features are not preferred for sexual reproduction.

• Peripatric speciation – Speciation in a small, isolated, peripheral population. • Phyletic Gradualism – A model that theorizes that speciation is gradual, incremental and slow. • Saltation – Sudden change that occurs over the span of a single generation.