Punctuated Equilibrium Definition

Punctuated equilibrium is a hypothesis that says that evolution happens in brief bursts of intensive speciation followed by long periods of stability or equilibrium. According to the model, approximately 99 percent of a species’ life on Earth is spent in stasis while change occurs at a rapid pace.

Punctuated Equilibrium Explained

So, if a species has been recorded in fossil records for around 10 million years, speciation is most likely to have happened in the last 100,000 years. However, once completed, the model implies that following the speciation event, there is minimal morphological change. The species will subsequently enter a phase of stability known as stasis, which will last for a long time.

The hypothesis also explains why there are no intermediate forms in fossil records, when new species come out of nowhere from ancestral forms and then vanish without undergoing any morphological change.

While this was a departure from the assumption that all new species formed as a result of slow, incremental changes, the theory’s creators also admitted that different processes of evolution may coexist. Gradualism, on the other hand, proposes that change happens gradually and over lengthy periods of time.

Punctuated Equilibrium Examples

In fossil records, evidence for punctuated equilibrium may be found. Biologists, systematists, and taxonomists from all over the globe have studied the effects of reproductive isolation. Because this is an evolutionary hypothesis, its predictions cannot be directly tested.

While the fossil record may support the notion, some evidence from the current world is also required. Animals living in identical settings that undergo reproductive isolation, for example, must be unable to interbreed, suggesting the formation of a new species. This has been seen on multiple occasions.

Reproductive Isolation among Kingfishers

The influence of reproductive isolation on speciation was shown in a study of kingfishers in Papua, New Guinea. On the mainland, there are three different subspecies. The climate here may range from humid, lush rain forests to monsoon forests with long dry seasons. Not only may these subspecies interbreed, but they are practically indistinguishable from one another.

However, even when the ecology is comparable to that of the closest area of the mainland, the kingfishers on islands a few hundred kilometres away are noticeably different. On these tiny islands, more species have been discovered than on the mainland. Similar findings have been obtained for birds, reptiles, and invertebrates all across the globe, where geographical isolation has resulted in the creation of new species, whereas broad continuous tracts with diverse environments retain homogenous populations. The punctuated equilibrium hypothesis is supported by this data.

Land Snails of Bermuda

Some 300,000 years ago, Poecilozonites bermudensis, an air-breathing land snail, entered Bermuda. The snail might have travelled from North America on driftwood. These snails’ fossils make up the vast bulk of Bermuda’s terrestrial fossils.

Until recently, the island was home to just one species. This snail has two stocks with different colour banding patterns in their early populations. When they became extinct, a descendant of a peripheral population developing on a distant island took over as the dominant species. The recurrent development of species from peripherally separated populations eventually led to the emergence of the land snail, which remained morphologically unchanging until it was discovered in the 1950s, according to fossil samples from six distinct geological eras and varied geographical regions.

Features of Punctuated Equilibrium

The need for reproductive isolation in the development of new species is one of the pillars of this idea. Because new species may only arise from tiny, isolated populations, the fossil record at any one location is unlikely to document the process of speciation. As a result, differences will only be detected in fossils of the same age from different geographical regions.

Rapid Morphological Changes

Punctuated equilibrium states that in small populations, genetic and physical changes that provide a survival advantage will be amplified quickly. The quick rate of development in these isolated groups is also cited as an explanation for the lack of a fossil record of evolution and the sudden appearance of new species.

It also predicts that, whereas intermediates would be uncommon in single-species evolution, they would be common in larger groupings. While Australopithecus afarensis is the ancestor of modern humans, there are no remains of Australopithecus afarensis exhibiting a steady increase in brain capacity or body size. Other species, such as Homo habilis and Homo erectus, offer evidence of a transition from Australopithecus to modern man in terms of brain capacity and body size.

Periods of Stasis

The explanation offered for lengthy periods of stasis is another essential characteristic of this concept. It means that a species’ average morphology is becoming homogenised. Interbreeding populations seem to be in a state of equilibrium. Small changes within a big population get diluted and homogenised, which is why this happens.

A variety of theories have been proposed to explain this anomaly in the fossil record. The impact of gene flow, claims that a species’ morphology is under “homeostatic” pressure, and koinophilia, or the rejection of mates with atypical characteristics, are all examples.

Gradualism vs Punctuated Equilibrium

Phyletic gradualism is opposed to punctuated equilibrium. Both of these conflicting evolutionary theories postulate the pace at which new species evolve. Gradualism emphasises the gradual introduction of new features in interbreeding subspecies that contribute to the evolution of a new species from ancestral forms through time.

Throughout history, fossils have occurred out of nowhere. Punctuated equilibrium seeks to explain these fossils “gaps,” or the lack of intermediate forms, by claiming that they only exist for extremely brief periods of time when intensive speciation occurs in a single population.

Punctuated equilibrium is criticised for the possibility that fossil records are just incomplete. In areas where fossils are numerous and well-preserved, intermediate forms may be identified. Furthermore, opponents point out that there is no proof that an external homogenising force preserves interbreeding populations in a state of equilibrium.