Punctuated equilibrium - AbsoluteAstronomy.com

Punctuated equilibrium is a theory in evolutionary biology

Evolution

Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...

which proposes that most species

Species

In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

will exhibit little net evolutionary change for most of their geological history, remaining in an extended state called stasis. When significant evolutionary change occurs, the theory proposes that it is generally restricted to rare and geologically rapid

Geologic time scale

The geologic time scale provides a system of chronologic measurement relating stratigraphy to time that is used by geologists, paleontologists and other earth scientists to describe the timing and relationships between events that have occurred during the history of the Earth...

events of branching speciation called cladogenesis

Cladogenesis

Cladogenesis is an evolutionary splitting event in a species in which each branch and its smaller branches forms a "clade", an evolutionary mechanism and a process of adaptive evolution that leads to the development of a greater variety of sister species...

. Cladogenesis is the process by which a species splits into two distinct species, rather than one species gradually transforming into another.

Punctuated equilibrium is commonly contrasted against the theory of phyletic gradualism

Phyletic gradualism

Phyletic gradualism is a model of evolution which theorizes that most speciation is slow, uniform and gradual. When evolution occurs in this mode, it is usually by the steady transformation of a whole species into a new one...

, which states that evolution generally occurs uniformly and by the steady and gradual transformation of whole lineages (called anagenesis

Anagenesis

Anagenesis, also known as "phyletic change," is the evolution of species involving an entire population rather than a branching event, as in cladogenesis. When enough mutations have occurred and become stable in a population so that it is significantly differentiated from an ancestral population,...

). In this view, evolution is seen as generally smooth and continuous.

In 1972, paleontologists Niles Eldredge

Niles Eldredge

Niles Eldredge is an American paleontologist, who, along with Stephen Jay Gould, proposed the theory of punctuated equilibrium in 1972.-Education:...

and Stephen Jay Gould

Stephen Jay Gould

Stephen Jay Gould was an American paleontologist, evolutionary biologist, and historian of science. He was also one of the most influential and widely read writers of popular science of his generation....

published a landmark paper developing this theory and called it punctuated equilibria. Their paper built upon Ernst Mayr's

Ernst Mayr

Ernst Walter Mayr was one of the 20th century's leading evolutionary biologists. He was also a renowned taxonomist, tropical explorer, ornithologist, historian of science, and naturalist...

theory of geographic speciation

Allopatric speciation

Allopatric speciation or geographic speciation is speciation that occurs when biological populations of the same species become isolated due to geographical changes such as mountain building or social changes such as emigration...

, I. Michael Lerner

I. Michael Lerner

Isador Michael Lerner was a prominent geneticist and evolutionary biologist. Born in Harbin, Manchuria, he received his Ph.D. in genetics at the University of California, Berkeley in 1936...

's theories of developmental and genetic homeostasis, as well as their own empirical research

Empirical research

Empirical research is a way of gaining knowledge by means of direct and indirect observation or experience. Empirical evidence can be analyzed quantitatively or qualitatively...

. Eldredge and Gould proposed that the degree of gradualism commonly attributed to Charles Darwin

Charles Darwin

Charles Robert Darwin FRS was an English naturalist. He established that all species of life have descended over time from common ancestry, and proposed the scientific theory that this branching pattern of evolution resulted from a process that he called natural selection.He published his theory...

is virtually nonexistent in the fossil record, and that stasis dominates the history of most fossil

Fossil

Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...

species.

A history of the theory

Punctuated equilibrium originated as a logical extension of Ernst Mayr's

Ernst Mayr

Ernst Walter Mayr was one of the 20th century's leading evolutionary biologists. He was also a renowned taxonomist, tropical explorer, ornithologist, historian of science, and naturalist...

concept of genetic revolutions

Founder effect

In population genetics, the founder effect is the loss of genetic variation that occurs when a new population is established by a very small number of individuals from a larger population. It was first fully outlined by Ernst Mayr in 1942, using existing theoretical work by those such as Sewall...

by allopatric

Allopatric speciation

and especially peripatric speciation

Peripatric speciation

Peripatric and peripatry are terms from biogeography, referring to organisms whose ranges are closely adjacent but do not overlap, being separated where these organisms do not occur – for example a wide river or a mountain range. Such organisms are usually closely related Peripatric and...

as applied to the fossil record. Although some of the basic workings of the theory were proposed and identified by Mayr in 1954, historians of science

History of science

The history of science is the study of the historical development of human understandings of the natural world and the domains of the social sciences....

generally recognize the 1972 paper by Niles Eldredge

Niles Eldredge

Niles Eldredge is an American paleontologist, who, along with Stephen Jay Gould, proposed the theory of punctuated equilibrium in 1972.-Education:...

and Stephen Jay Gould

Stephen Jay Gould

as the foundational document of the new paleobiological

Paleobiology

Paleobiology is a growing and comparatively new discipline which combines the methods and findings of the natural science biology with the methods and findings of the earth science paleontology...

research program. Punctuated equilibrium differs from Mayr's hypothesis mainly in that Eldredge and Gould placed considerably greater emphasis on stasis, whereas Mayr was generally concerned with explaining the morphological

Morphology (biology)

In biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....

discontinuity (or "sudden jumps") found in the fossil record. Mayr later complimented Eldredge and Gould's paper, stating that evolutionary stasis had been "unexpected by most evolutionary biologists" and that punctuated equilibrium "had a major impact on paleontology and evolutionary biology."

A year before their 1972 Eldredge and Gould paper, Niles Eldredge

Niles Eldredge

Niles Eldredge is an American paleontologist, who, along with Stephen Jay Gould, proposed the theory of punctuated equilibrium in 1972.-Education:...

published a paper in the journal Evolution
Evolution (journal)
Evolution, the International Journal of Organic Evolution, is a leading monthly scientific journal that publishes significant new results of empirical or theoretical investigations concerning facts, processes, mechanics, or concepts of evolutionary phenomena and events. Evolution is published by...

which suggested that gradual evolution was seldom seen in the fossil record and argued that Ernst Mayr's standard mechanism of allopatric speciation

Allopatric speciation

might suggest a possible resolution.

The Eldredge and Gould paper was presented at the Annual Meeting of the Geological Society of America

Geological Society of America

The Geological Society of America is a nonprofit organization dedicated to the advancement of the geosciences. The society was founded in New York in 1888 by Alexander Winchell, John J. Stevenson, Charles H. Hitchcock, John R. Proctor and Edward Orton and has been headquartered at 3300 Penrose...

in 1971. The symposium focused its attention on the possibility that modern microevolution

Microevolution

Microevolution is the changes in allele frequencies that occur over time within a population. This change is due to four different processes: mutation, selection , gene flow, and genetic drift....

ary studies could revitalize various aspects of paleontology and macroevolution. Tom Schopf, who organized that year's meeting, assigned Gould the topic of speciation. Gould recalls that "Eldredge's 1971 publication [on Paleozoic

Paleozoic

The Paleozoic era is the earliest of three geologic eras of the Phanerozoic eon, spanning from roughly...

trilobite

Trilobite

Trilobites are a well-known fossil group of extinct marine arthropods that form the class Trilobita. The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period , and they flourished throughout the lower Paleozoic era before...

s] had presented the only new and interesting ideas on the paleontological implications of the subject—so I asked Schopf if we could present the paper jointly."
According to Gould "the ideas came mostly from Niles, with yours truly acting as a sounding board and eventual scribe. I coined the term punctuated equilibrium and wrote most of our 1972 paper, but Niles is the proper first author in our pairing of Eldredge and Gould." Eldredge in his book Time Frames recalls that the pair "after much discussion, each wrote roughly half. Some of the parts that would seem obviously the work of one of us were actually first penned by the other—I remember for example, writing the section on Gould's snails. Other parts are harder to reconstruct. Gould edited the entire manuscript for better consistency. We sent it in, and Schopf reacted strongly against it—thus signaling the tenor of the reaction it has engendered, though for shifting reasons, down to the present day."

Punctuational change

When Eldredge and Gould published their 1972 paper, allopatric speciation

Allopatric speciation

was considered the "standard" theory of speciation. This theory was popularized by Ernst Mayr in his 1954 paper "Change of genetic environment and evolution," and his classic volume Animal Species and Evolution (1963).

Allopatric speciation suggests that species with large central populations are stabilized by their large volume and the process gene flow

Gene flow

In population genetics, gene flow is the transfer of alleles of genes from one population to another.Migration into or out of a population may be responsible for a marked change in allele frequencies...

. New and even beneficial mutations are diluted by the population's large size and are unable to reach fixation, due to such factors as constantly changing environments. If this is the case, then the transformation of whole lineages should be rare, as the fossil record indicates. Smaller populations on the other hand, which are isolated from the parental stock, are decoupled from the homogenizing effects of gene flow. In addition, pressure from natural selection

Natural selection

Natural selection is the nonrandom process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers. It is a key mechanism of evolution....

is especially intense, as peripheral isolated populations exist at the outer edges of ecological tolerance. If most evolution happens in these rare instances of allopatric speciation then evidence of gradual evolution in the fossil record should be rare. This stimulating hypothesis

Hypothesis

A hypothesis is a proposed explanation for a phenomenon. The term derives from the Greek, ὑποτιθέναι – hypotithenai meaning "to put under" or "to suppose". For a hypothesis to be put forward as a scientific hypothesis, the scientific method requires that one can test it...

was alluded to by Mayr in the closing paragraph of his 1954 paper (p. 179).

As time went on Gould moved away from wedding punctuated equilibrium to allopatric speciation, particularly as evidence accumulated in support of other modes of speciation. Gould was particularly attracted to Douglas Futuyma's

Douglas J. Futuyma

Douglas Joel Futuyma is an American biologist.-Academics:Futuyma graduated with a B.S. from Cornell University, and took his M.S. and Ph.D. at the University of Michigan. His research focuses on the interaction between plant-eating insects and the plants themselves. He was Lawrence B...

work on the importance of reproductive isolating mechanisms.

Other biologists have also applied punctuated equilibrium to non-sexual species, including the evolution of virus

Virus

A virus is a small infectious agent that can replicate only inside the living cells of organisms. Viruses infect all types of organisms, from animals and plants to bacteria and archaea...

es.

Stasis

Before Eldredge and Gould alerted their colleagues to the prominence of stasis in the fossil record, most evolutionists considered stasis to be rare or unimportant. George Gaylord Simpson for example believed that phyletic gradual evolution (called horotely in his terminology) comprised "nine-tenths" (90%) of evolution. Many hypotheses have been proposed to explain the putative causes of stasis. Gould was initially attracted to I. Michael Lerner

I. Michael Lerner

Isador Michael Lerner was a prominent geneticist and evolutionary biologist. Born in Harbin, Manchuria, he received his Ph.D. in genetics at the University of California, Berkeley in 1936...

's theories of developmental and genetic homeostasis. However this hypothesis was rejected over time, as evidence accumulated against it. Other plausible mechanisms which have been suggested include: habitat tracking, stabilizing selection

Stabilizing selection

-Description:Stabilizing or ambidirectional selection, , is a type of natural selection in which genetic diversity decreases as the population stabilizes on a particular trait value. This is probably the most common mechanism of action for natural selection...

, the Stenseth-Maynard Smith stability hypothesis, constraints imposed by the nature of subdivided populations, and normalizing clade selection.

Evidence for the existence of stasis has also been corroborated from the genetics of sibling species

Sibling species

Sibling species are species that are very similar in appearance, in behavior and in other characteristics, but they are reproductively isolated. In other words, sibling species are pairs or groups of genetically closely related species which are often morphologically indistinguishable, but are...

, species which are morphologically indistinguishable, but whose proteins have diverged sufficiently to suggest they have been separated for millions of years. According to Gould "stasis may emerge as the theory's most important contribution to evolutionary science."

Philosopher Kim Sterelny

Kim Sterelny

Kim Sterelny is an Australian philosopher and professor of philosophy in the Research School of Social Sciences at Australian National University and Victoria University of Wellington. He is the winner of several international prizes in the philosophy of science, and editor of Biology and Philosophy...

adds, "In claiming that species typically undergo no further evolutionary change once speciation is complete, they are not claiming that there is no change at all between one generation and the next. Lineages do change. But the change between generations does not accumulate. Instead, over time, the species wobbles about its phenotypic mean. Jonathan Weiner's

Jonathan Weiner

Jonathan Weiner is a Pulitzer Prize-winning author of non-fiction books on his biology observations, in particular evolution in the Galápagos Islands, genetics, and the environment....

The Beak of the Finch
The Beak of the Finch
The Beak of the Finch: A Story of Evolution in Our Time is a 1994 nonfiction book about evolutionary biology, written by Jonathan Weiner. It won the 1995 Pulitzer Prize for General Non-Fiction. The finches of the title are the Galapagos or 'Darwin's Finches,' passerine songbirds in the Galapagos...

describes this very process."

The fossil record includes well documented examples of phyletic gradualism and punctuational evolution. As such, much debate persist over the prominence of stasis in the fossil record.

Hierarchical evolution

Punctuated equilibrium has also been cited as contributing to the theory that species are Darwinian individuals

Unit of selection

A unit of selection is a biological entity within the hierarchy of biological organisation that is subject to natural selection...

, and not just classes

Class (biology)

In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...

, thereby providing a stronger framework for a hierarchical theory of evolution.

Common misconceptions

Much confusion has arisen over what proponents of punctuated equilibrium actually argued, what mechanisms they advocated, how fast the punctuations were, what taxonomic scale their theory applied to, how revolutionary their claims were intended to be, and how punctuated equilibrium related to other ideas like quantum evolution

Quantum evolution

Quantum evolution is a component of George Gaylord Simpson's multi-tempoed theory of evolutionary change, proposed to explain the rapid emergence of higher taxonomic groups. According to Simpson, evolutionary rates differ from group to group and even among closely related lineages...

, saltationism

Saltation (biology)

In biology, ""saltation"" is a sudden change from one generation to the next, that is large, or very large, in comparison with the usual variation of an organism...

, and mass extinction.

Saltationism

The punctuational nature of punctuated equilibrium has engendered perhaps the most confusion over Eldredge and Gould's theory. Gould's sympathetic treatment of Richard Goldschmidt

Richard Goldschmidt

Richard Benedict Goldschmidt was a German-born American geneticist. He is considered the first to integrate genetics, development, and evolution. He pioneered understanding of reaction norms, genetic assimilation, dynamical genetics, sex determination, and heterochrony...

, the controversial geneticist

Genetics

Genetics , a discipline of biology, is the science of genes, heredity, and variation in living organisms....

who advocated the idea of "hopeful monsters," only exacerbated the matter, which lead some biologists to conclude that Gould's punctuations were occurring in single-generation jumps. This interpretation has frequently been exploited by creationists

Creationism

Creationism is the religious beliefthat humanity, life, the Earth, and the universe are the creation of a supernatural being, most often referring to the Abrahamic god. As science developed from the 18th century onwards, various views developed which aimed to reconcile science with the Genesis...

to mischaracterize the weakness of the paleontological

Paleontology

Paleontology "old, ancient", ὄν, ὀντ- "being, creature", and λόγος "speech, thought") is the study of prehistoric life. It includes the study of fossils to determine organisms' evolution and interactions with each other and their environments...

record, and to portray contemporary evolutionary biology as advancing neo-saltationism. In an often quoted remark, Gould stated, "Since we proposed punctuated equilibria to explain trends, it is infuriating to be quoted again and again by creationists—whether through design or stupidity, I do not know—as admitting that the fossil record includes no transitional forms

Transitional fossil

A transitional fossil is any fossilized remains of a lifeform that exhibits characteristics of two distinct taxonomic groups. A transitional fossil is the fossil of an organism near the branching point where major individual lineages diverge...

. Transitional forms are generally lacking at the species level, but they are abundant between larger groups." Although there exist some debate over how long the punctuations last, supporters of punctuated equilibrium generally place the figure between 50,000 and 100,000 years.

Quantum evolution

was a controversial hypothesis advanced by Columbia University

Columbia University

Columbia University in the City of New York is a private, Ivy League university in Manhattan, New York City. Columbia is the oldest institution of higher learning in the state of New York, the fifth oldest in the United States, and one of the country's nine Colonial Colleges founded before the...

paleontologist George Gaylord Simpson

George Gaylord Simpson

George Gaylord Simpson was an American paleontologist. Simpson was perhaps the most influential paleontologist of the twentieth century, and a major participant in the modern evolutionary synthesis, contributing Tempo and mode in evolution , The meaning of evolution and The major features of...

, who was regarded by Stephen Jay Gould as "the greatest and most biologically astute paleontologist of the twentieth century." Simpson's conjecture was that according to the geological record, on very rare occasions evolution would proceed very rapidly to form entirely new families

Family (biology)

In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...

, orders

Order (biology)

In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...

, and classes

Class (biology)

In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...

of organisms. This hypothesis differs from punctuated equilibrium in many respects. First, punctuated equilibrium was much more modest in scope, in that it was specifically addressing evolution specifically at the species

Species

level. Simpson's idea was principally concerned with evolution at higher taxonomic groups. Second, Eldredge and Gould relied upon an entirely different mechanism. Where Simpson relied upon a synergistic

Synergy

Synergy may be defined as two or more things functioning together to produce a result not independently obtainable.The term synergy comes from the Greek word from , , meaning "working together".-Definitions and usages:...

interaction between genetic drift

Genetic drift

Genetic drift or allelic drift is the change in the frequency of a gene variant in a population due to random sampling.The alleles in the offspring are a sample of those in the parents, and chance has a role in determining whether a given individual survives and reproduces...

and a shift in the adaptive fitness landscape

Fitness landscape

In evolutionary biology, fitness landscapes or adaptive landscapes are used to visualize the relationship between genotypes and reproductive success. It is assumed that every genotype has a well-defined replication rate . This fitness is the "height" of the landscape...

, Eldredge and Gould relied upon ordinary speciation, particularly Ernst Mayr's concept of allopatric speciation. Lastly, and perhaps most significantly, quantum evolution took no position on the issue of stasis. Although Simpson allowed for the existence of stasis in what he called the bradytelic mode, Simpson considered it (along with rapid evolution) to be insignificant in the larger scope of evolution. In his Major Features of Evolution Simpson stated, "Evolutionary change is so nearly the universal rule that a state of motion is, figuratively, normal in evolving populations. The state of rest, as in bradytely, is the exception and it seems that some restraint or force must be required to maintain it." Despite such differences between the two models, earlier critiques—from such eminent commentators as Sewall Wright

Sewall Wright

Sewall Green Wright was an American geneticist known for his influential work on evolutionary theory and also for his work on path analysis. With R. A. Fisher and J.B.S. Haldane, he was a founder of theoretical population genetics. He is the discoverer of the inbreeding coefficient and of...

and G. G. Simpson—have argued that punctuated equilibrium is little more than quantum evolution relabeled.

The multiple meanings of gradualism

Punctuated equilibrium is often portrayed to oppose the concept of gradualism, when it is actually a form of gradualism. This is because even though evolutionary change appears instantaneous between geological sediments, change is still occurring incrementally, with no great change from one generation to the next. To this end, Gould later commented that "Most of our paleontological colleagues

Paleontology

missed this insight because they had not studied evolutionary theory and either did not know about allopatric speciation

Allopatric speciation

or had not considered its translation to geological time. Our evolutionary colleagues

Population genetics

Population genetics is the study of allele frequency distribution and change under the influence of the four main evolutionary processes: natural selection, genetic drift, mutation and gene flow. It also takes into account the factors of recombination, population subdivision and population...

also failed to grasp the implication(s), primarily because they did not think at geological scales".

Richard Dawkins

Richard Dawkins

Clinton Richard Dawkins, FRS, FRSL , known as Richard Dawkins, is a British ethologist, evolutionary biologist and author...

dedicated a chapter in The Blind Watchmaker
The Blind Watchmaker
The Blind Watchmaker: Why the Evidence of Evolution Reveals a Universe without Design is a 1986 book by Richard Dawkins in which he presents an explanation of, and argument for, the theory of evolution by means of natural selection. He also presents arguments to refute certain criticisms made on...

to correcting, in his view, the wide confusion regarding rates of change. His first point is to argue that phyletic gradualism

Phyletic gradualism

— understood in the sense that evolution proceeds at a single uniform rate of speed, called "constant speedism" by Dawkins — is a "caricature of Darwinism" and "does not really exist." His second argument, which follows from the first, is that once the caricature of "constant speedism" is dismissed, we are left with one logical alternative, which Dawkins terms "variable speedism." Variable speedism may also be distinguished one of two ways: "discrete variable speedism" and "continuously variable speedism." Eldredge and Gould, believing that evolution jumps between stability and relative rapidity, are described as "discrete variable speedists," and "in this respect they are genuinely radical." They believe that evolution generally proceeds in bursts, or not at all. "Continuously variable speedists," on the other hand believe that "evolutionary rates fluctuate continuously from very fast to very slow and stop, with all intermediates. They see no particular reason to emphasize certain speeds more than others. In particular, stasis, to them, is just an extreme case of ultra-slow evolution. To a punctuationist, there is something very special about stasis." Dawkins therefore commits himself here to an empirical claim about the geological record, in contrast to his earlier claim that, "The paleontological evidence can be argued about, and I am not qualified to judge it." It is this particular commitment that Eldredge and Gould have aimed to overturn.

The relationship between punctuationism and gradualism can be better appreciated by considering an example. Suppose the average length of a limb in a particular species grows 50 centimeters (20 inches) over 70,000 years—a large amount in a geologically short period of time. If the average

Arithmetic mean

In mathematics and statistics, the arithmetic mean, often referred to as simply the mean or average when the context is clear, is a method to derive the central tendency of a sample space...

generation is seven years, then our given time span corresponds to 10,000 generations. It is therefore reasonable to conclude that if the limb size in our hypothetical population evolved in the most conservative manner, it need only increase at a rate of 0.005 cm per generation (= 50 cm/10,000), despite its abrupt appearance in the geological record.

Criticism

Richard Dawkins

Clinton Richard Dawkins, FRS, FRSL , known as Richard Dawkins, is a British ethologist, evolutionary biologist and author...

believes that the apparent gaps represented in the fossil record document migratory events rather than evolutionary events. According to Dawkins, evolution certainly occurred but "probably gradually" elsewhere. However, the punctuational equilibrium model may still be inferred from both the observance of stasis and documented examples of rapid and episodic speciation events documented in the fossil record.

Dawkins also emphasizes that punctuated equilibrium has been "oversold by some journalists", but partly due to Eldredge and Gould's "later writings". Dawkins contends that the theory "does not deserve a particularly large measure of publicity". It is a "minor gloss," an "interesting but minor wrinkle on the surface of neo-Darwinian theory," and "lies firmly within the neo-Darwinian synthesis".

In his book Darwin's Dangerous Idea
Darwin's Dangerous Idea
Darwin's Dangerous Idea: Evolution and the Meanings of Life is a book by Daniel Dennett which argues that Darwinian processes are the central organizing force that gives rise to complexity...

, philosopher Daniel Dennett

Daniel Dennett

Daniel Clement Dennett is an American philosopher, writer and cognitive scientist whose research centers on the philosophy of mind, philosophy of science and philosophy of biology, particularly as those fields relate to evolutionary biology and cognitive science. He is currently the Co-director of...

is especially critical of Gould's presentation of punctuated equilibrium. Dennett argues that Gould alternated between revolutionary and conservative claims about the theory, and that each time Gould made a revolutionary statement—or appeared to do so—he was criticized, and thus retreated to a traditional neo-Darwinian position. Gould responded to Dennett's claims in The New York Review of Books
The New York Review of Books
The New York Review of Books is a fortnightly magazine with articles on literature, culture and current affairs. Published in New York City, it takes as its point of departure that the discussion of important books is itself an indispensable literary activity...

, and in his technical volume The Structure of Evolutionary Theory
The Structure of Evolutionary Theory
The Structure of Evolutionary Theory is a technical book on macroevolutionary theory by the Harvard paleontologist Stephen Jay Gould, published only two months before his death. The volume is divided into two parts. The first is a historical study and exegesis of classical evolutionary thought,...

.

Literary scholar Heidi Scott argued that Gould's use of analogy and metaphor constitutes a non-scientific discourse serving to validate a scientific theory. She claims that Gould—particularly in his popular essays—uses a variety of strategies from literature, political science, and personal anecdotes to substantiate the general pattern of punctuated equilibrium (long periods of stasis interrupted by rapid, catastrophic change). Gould responded that critics often made the mistake of confusing the context of discovery with the context of justification. While Gould is celebrated for the color and energy of his prose, as well as his massive interdisciplinary knowledge, critics such as Scott have concerns that the theory has gained undeserved credence among non-scientists because of Gould's rhetorical skills.

Randy Allen Harris, Professor of Rhetoric at the University of Waterloo

University of Waterloo

The University of Waterloo is a comprehensive public university in the city of Waterloo, Ontario, Canada. The school was founded in 1957 by Drs. Gerry Hagey and Ira G. Needles, and has since grown to an institution of more than 30,000 students, faculty, and staff...

, in a more positive evaluation, states that "re-analysis of existing fossil data has shown, to the increasing satisfaction of the paleontological community, that Eldredge and Gould were correct in identifying periods of evolutionary stasis which are interrupted by much shorter periods of evolutionary change."

Relation to Darwin's theories

The sudden appearance of most species in the geologic record and the lack of evidence of substantial gradual change in most species—from their initial appearance until their extinction—has long been noted, including by Charles Darwin

Charles Darwin

who appealed to the imperfection of the record as the favored explanation. When presenting his ideas against the prevailing influences of catastrophism

Catastrophism

Catastrophism is the theory that the Earth has been affected in the past by sudden, short-lived, violent events, possibly worldwide in scope. The dominant paradigm of modern geology is uniformitarianism , in which slow incremental changes, such as erosion, create the Earth's appearance...

and progressive creationism

Progressive creationism

Progressive creationism is the religious belief that God created new forms of life gradually, over a period of hundreds of millions of years. As a form of Old Earth creationism, it accepts mainstream geological and cosmological estimates for the age of the Earth, but posits that the new "kinds" of...

, which envisaged species being supernaturally created at intervals, Darwin needed to forcefully stress the gradual nature of evolution

Evolution

in accordance with the gradualism

Gradualism

Gradualism is the belief in or the policy of advancing toward a goal by gradual, often slow stages.-Politics and society:In politics, the concept of gradualism is used to describe the belief that change ought to be brought about in small, discrete increments rather than in abrupt strokes such as...

promoted by his friend Charles Lyell

Charles Lyell

Sir Charles Lyell, 1st Baronet, Kt FRS was a British lawyer and the foremost geologist of his day. He is best known as the author of Principles of Geology, which popularised James Hutton's concepts of uniformitarianism – the idea that the earth was shaped by slow-moving forces still in operation...

. He privately expressed concern, noting in the margin of his 1844 Essay, "Better begin with this: If species really, after catastrophes, created in showers world over, my theory false."

It is often incorrectly assumed that he insisted that the rate of change must be constant, or nearly so, but even the first edition of On the Origin of Species states that "Species of different genera and classes have not changed at the same rate, or in the same degree. In the oldest tertiary beds a few living shells may still be found in the midst of a multitude of extinct forms... The Silurian Lingula differs but little from the living species of this genus". Lingula

Lingula (genus)

Lingula is a genus of brachiopods within the class Lingulata. Lingula is known since the Tertiary.-Species:The following species are recognised:*Lingula adamsi Dall, 1873*Lingula anatina Lamarck, 1801*Lingula dregeri Andreae, 1893...

is among the few brachiopods surviving today but also known from fossils over 500 million years old. In the fifth edition of On the Origin of Species Darwin wrote that "the periods during which species have undergone modification, though long as measured in years, have probably been short in comparison with the periods during which they retain the same form." Thus punctuationism in general is consistent with Darwin's conception of evolution.

According to early versions of punctuated equilibrium, "peripheral isolates" are considered to be of critical importance for speciation. However, Darwin wrote, "I can by no means agree ... that immigration and isolation are necessary elements.... Although isolation is of great importance in the production of new species, on the whole I am inclined to believe that largeness of area is still more important, especially for the production of species which shall prove capable of enduring for a long period, and of spreading widely."

The importance of isolation in forming species had played a significant part in Darwin's early thinking, as shown in his Essay of 1844. But by the time he wrote the Origin he had downplayed its importance. He explained the reasons for his revised view as follows:

Throughout a great and open area, not only will there be a greater chance of favourable variations, arising from the large number of individuals of the same species there supported, but the conditions of life are much more complex from the large number of already existing species; and if some of these species become modified and improved, others will have to be improved in a corresponding degree, or they will be exterminated. Each new form, also, as soon as it has been improved, will be able to spread over the open and continuous area, and will thus come into competition with many other forms ... the new forms produced on large areas, which have already been victorious over many competitors, will be those that will spread most widely, and will give rise to the greatest number of new varieties and species. They will thus play a more important role in the changing history of the organic world.

Thus punctuated equilibrium contradicts some of Darwin's ideas regarding the specific mechanisms of evolution, but generally accords with Darwin's theory of evolution by natural selection.

Supplemental modes of rapid evolution

Recent work in developmental biology

Developmental biology

Developmental biology is the study of the process by which organisms grow and develop. Modern developmental biology studies the genetic control of cell growth, differentiation and "morphogenesis", which is the process that gives rise to tissues, organs and anatomy.- Related fields of study...

has identified dynamical and physical mechanisms of tissue morphogenesis

Morphogenesis

Morphogenesis , is the biological process that causes an organism to develop its shape...

that may underlie abrupt morphological transitions during evolution. Consequently, consideration of mechanisms of phylogenetic change that have been found in reality to be non-gradual is increasingly common in the field of evolutionary developmental biology

Evolutionary developmental biology

Evolutionary developmental biology is a field of biology that compares the developmental processes of different organisms to determine the ancestral relationship between them, and to discover how developmental processes evolved...

, particularly in studies of the origin of morphological novelty. A description of such mechanisms can be found in the multi-authored volume Origination of Organismal Form
Origination of Organismal Form
Origination of Organismal Form: Beyond the Gene in Developmental and Evolutionary Biology is a book published in 2003 edited by Gerd B. Müller and Stuart A. Newman. It explores the multiple factors that may have been responsible for the origination of biological form in multicellular life...

(MIT Press; 2003).

Punctuated equilibrium in social theory

Social theory

Social theories are theoretical frameworks which are used to study and interpret social phenomena within a particular school of thought. An essential tool used by social scientists, theories relate to historical debates over the most valid and reliable methodologies , as well as the primacy of...

is a method of understanding change in complex social systems, particularly how policy change and the development of conflicts seem to progress in extended periods of stasis, punctuated by sudden shifts in radical change.

Punctuated equilibrium in language change

In linguistics, R.M.W. Dixon has proposed a puncuated equilibrium model for language histories, with reference particularly to the prehistory of the indigenous languages of Australia and his objections to the proposed Pama–Nyungan language family there. Although his model has raised considerable interest, it does not command majority support within linguistics. Separately, recent work using computational phylogenetic

Computational phylogenetics

Computational phylogenetics is the application of computational algorithms, methods and programs to phylogenetic analyses. The goal is to assemble a phylogenetic tree representing a hypothesis about the evolutionary ancestry of a set of genes, species, or other taxa...

methods claims to show that punctuational bursts play an important factor when languages split from one another

Language change

Language change is the phenomenon whereby phonetic, morphological, semantic, syntactic, and other features of language vary over time. The effect on language over time is known as diachronic change. Two linguistic disciplines in particular concern themselves with studying language change:...

, accounting for anywhere from 10 to 33% of the total divergence in vocabulary.

External links

Opus 200 - by Stephen Jay Gould
Punctuated Equilibrium - by Stephen Jay Gould
Punctuated Equilibria: An Alternative to Phyletic Gradualism - by Niles Eldredge and Stephen Jay Gould
Punctuated Equilibria: The Tempo and Mode of Evolution Reconsidered - by Stephen Jay Gould and Niles Eldredge
Speciational Evolution or Punctuated Equilibria - by Ernst Mayr
Punctuated Equilibrium Comes of Age - by Stephen Jay Gould and Niles Eldredge
Punctuated Equilibria—Where is the Evidence? - by Philip D. Gingerich
Punctuated Equilibrium at Twenty - by Donald R. Prothero
On Punctuated Equilibria - by Niles Eldredge and Stephen Jay Gould
Punctuated Equilibrium's Threefold History - by Stephen Jay Gould
The Dynamics of Evolutionary Stasis - by Niles Eldredge
Punctuated Equilibria? - by Wesley Elsberry, TalkOrigins Archive
TalkOrigins Archive
The TalkOrigins Archive is a website that presents mainstream science perspectives on the antievolution claims of young-earth, old-earth, and "intelligent design" creationists...
Scholarpedia: Punctuated equilibria - by Bruce Lieberman and Niles Eldredge
All you need to know about Punctuated Equilibrium (almost) - by Douglas Theobald

The source of this article is wikipedia, the free encyclopedia. The text of this article is licensed under the GFDL.

A history of the theory

Punctuational change

Stasis

Hierarchical evolution

Common misconceptions

Saltationism

Quantum evolution

The multiple meanings of gradualism

Criticism

Relation to Darwin's theories

Supplemental modes of rapid evolution

Punctuated equilibrium in social theory

Punctuated equilibrium in language change

See also

Further reading

External links