The punctuated equilibrium theory or punctualism, in evolutionary biology, seeks to explain the pattern of "jumps" of the fossil record in the process of formation of new species. One of the crucial controversies in evolution is related to leaps in the fossil record: are these morphological gaps due to gaps in the record (which is obviously incomplete) or because evolution certainly occurs in leaps?
The theory of punctuated equilibrium supports the existence of periods of stasis or periods of morphological stability, followed by rapid and abrupt events of evolutionary changes..
It was proposed in 1972 by the famous evolutionary biologist and paleontologist Stephen Jay Gould and his colleague Niles Eldrege. In this famous essay, the authors claim that paleontologists have misinterpreted neo-Darwinism.
Eldredge and Gould distinguish two extreme hypotheses about the patterns of changes that occur in evolutionary time.
The first is phyletic gradualism, where evolution occurs at a constant rate. In this case, the species are formed through a process of gradual transformation starting from the ancestral species and the rate of evolution during the speciation process is similar to any other moment..
The authors contrast the other extreme of the evolutionary rates with their own hypothesis: the punctuated equilibrium.
The influential essay by Eldredge and Gould includes the phenomena of stasis and the sudden or instantaneous appearance of forms in the normal process of speciation, that is, the formation of new species..
For defenders of punctuated equilibrium, periods of stasis are the normal condition of a species, which is only broken when the speciation event occurs (the moment where all evolutionary change is concentrated). Therefore, any change event outside the speciation event contradicts the theory.
The theory integrates the allopatric speciation model to discuss the reason why the fossil record should exhibit a differential pattern to that proposed by phyletic gradualists..
In the event that a species originates through the allopatric model and also, in small populations, the fossil record would not have to show the speciation process. In other words, the species do not have to originate in the same geographic region where the ancestral form inhabited..
The new species will only leave a trace in the same area as the ancestral species, only if it is able to invade the area again, in a post-speciation event. And for this to happen, reproductive barriers must be formed to prevent hybridization..
Therefore, we should not expect to find forms of transition. Not only because the record is incomplete, but because the speciation process took place in another region.
The term stasis refers to colossal periods of time where species do not undergo significant morphological changes. Upon careful analysis of the registry, this pattern has become apparent.
Innovations in evolution seemed to emerge alongside the process of speciation, and the tendency is to stay that way for a few million years..
Thus, periods of stasis are interrupted by instantaneous speciation events (in geological time). Although gradual transitions have been documented, this pattern does not appear to be the rule..
The British naturalist Charles Darwin was aware of this phenomenon, and in fact captured it in his masterpiece The origin of species.
A phenomenon as extraordinary as periods of stasis must have an explanation that is adjusted to the magnitude of the event. Many biologists have wondered why there are considerable periods of time where morphology remains constant, and various hypotheses have attempted to explain this evolutionary event.
An attempt has been made to elucidate the problem using living fossils as a model organism - species or clades whose changes have been undetectable or minimal over time..
An example of a living fossil is the genus Limulus, commonly known as casserole crab. The current species are extremely similar to the fossils of the family that date back more than 150 million years.
Some researchers suggested that the groups could lack genetic variation that promoted morphological change. However, subsequent genetic research showed that the variation is comparable to close groups of arthropods that vary as the average forms..
Theoretically, the most parsimonious explanation is the action of the stabilizing selection model, where the average morphology is favored and the rest is eliminated from the population over the generations. However, there are criticisms of this explanation, mainly due to the marked environmental changes.
In the fossil record, the evidence is inconclusive, since there are groups or lineages that support the theory of punctuated equilibrium, while others are a clear example of phyletic gradualism..
The bryozoans of the Caribbean are a group of marine invertebrates that show a pattern of evolution congruent with what is suggested by the punctuated equilibrium. In contrast, the studied trilobites exhibit a gradual change.
Punctuated equilibrium has been debated by evolutionary biologists and has sparked enormous controversy in the field. The main criticisms are the following:
According to some authors (such as Freeman & Herron, for example), discrepancies occur due to differences in the time scale. Biologists and paleontologists generally do not work at comparable time scales.
On scales of years or decades, gradual changes and natural selection seem to dominate, while on geological scales that span millions of years, sudden changes seem instantaneous..
Furthermore, the controversy is difficult to resolve due to the experimental difficulties involved in comparing punctuated equilibrium with phyletic gradualism..
Punctuated equilibrium is said to contradict the fundamental principles of the Darwinian theory of evolution. This idea comes from the misinterpretation of the term gradual by the parents of the theory.
In evolutionary biology, the term gradual can be used in two senses. One to explain constant evolutionary rates (phyletic gradualism); while the second meaning refers to the process of formation of adaptations, particularly the most complex ones - such as the eye.
In this sense, adaptations do not arise instantaneously and this concept is a crucial requirement in Darwinian theory of evolution. However, the first meaning of the term gradual is not a requirement of Darwinian theory..
Gould wrongly concluded that his theory contradicted Darwin's ideas, because he understood the term "gradual" in its first definition - whereas Darwin used it in terms of adaptations..
Finally, the theory involves controversial models of speciation, which makes it even more difficult to accept the punctuated equilibrium..
In particular, the idea that exposes the existence of two "valleys" and the intermediate form with a fitness less. This model was very popular in the 70s, when the authors published their ideas.