Macroevolution

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Macroevolution refers to evolution that occurs above the level of species, such as the origin of new designs (feathers, vertebrates from invertebrates, jaws in fish), large scale events (extinction of dinosaurs), broad trends (increase in brain size in mammals), and major transitions (origin of higher-level phyla). This is one of two classes of evolutionary phenomena, the other being microevolution, which refers to events and processes at or below the level of species, such as changes of gene frequencies in a population and speciation phenomena.

At times, the concept of macroevolution has been defined as including evolutionary change at and above the level of species, and microevolution below the level of species. As the dividing point, the process of speciation may be viewed variously as the purview of either macroevolution or microevolution.

Macroevolution is an autonomous field of evolutionary inquiry (Mayr 2001). Paleontology, evolutionary developmental biology, comparative genomics, and molecular biology contribute many advances relating to the patterns and processes that can be classified as macroevolution.

Since the time of Darwin, the concept of macroevolution has engendered controversy. The conventional view of many evolutionists is that macroevolution is simply a continuation of microevolution on a greater scale. Others see macroevolution as more or less decoupled from microevolution. This later perspective is held both by some prominent evolutionists, as well as by many religious adherents outside the scientific community. For example, movements such as creationism and intelligent design differentiate between microevolution and macroevolution, asserting that the former (change within a species) is an observable phenomena, but that the latter is not. Proponents of intelligent design argue that the mechanisms of evolution are incapable of giving rise to instances of specified complexity and irreducible complexity, and that while natural selection can be a creative force at the microevolutionary level, there is a divine power that is responsible as the creative force for macroevolutionary changes.

Contents

Overview

There are two views of macroevolution: (1) That it is simply an extension of microevolutionary processes over large time scales, and (2) that it is disconnected from microevolution, or involves different processes, such as punctuational change and species selection. Some evolutionary biologists, particularly Charles Darwin and those subscribing to the modern synthesis, see the only difference between microevolution and macroevolution as being one of scale. Other evolutionary biologists, including Gould, Schmalhausen, Stanley, and Waddington, hold that microevolution and macroevolution represent fundamentally different processes.

The historical view of macroevolution, which has held the status quo among evolutionists since Darwin, is that major trends, new designs, and other macroevolutionary events can be explained by extrapolation from microevolutionary processes. That is, it is inferred that if moths can change their color in 50 years, then new designs or entire new genera can originate over millions of years. If geneticists see population changes for fruit flies in laboratory bottles, then given eons of time, birds can be built from reptiles and fish with jaws from jawless ancestors. Macroevolution is simply the result of microevolution over a longer period of time. According to the modern synthesis, no distinction needs to be drawn between different kinds of evolution because all are caused by the same factors.

This conventional view rests on two central tenets: Gradualism and the primacy of natural selection (Luria, Gould, and Singer 1981). Gradualism is a view of evolution as proceeding by means of the slow accumulation of very small changes, with the evolving population passing through all the intermediate stages—sort of a "march of frequency distributions" through time. Natural selection is viewed as the causal agent of change. It is more than just removing unfit organisms, but actually directs the changes in gene frequencies, and at the higher levels, the development of new designs, major transitions, broad trends, and so forth. One of Darwin's chief purposes in publishing the Origin of Species was to show that natural selection had been the chief agent of the change presented in the theory of descent with modification.

Ever since Darwin elucidated his evolutionary theory, there has been controversy over whether macroevolution is simply an extrapolation or continuation of microevolution, or may be considered a separate phenomenon, involving different theories. Even today, the validity of making this extrapolation has come under strong challenge from top evolutionists, because of divergent views on these two central tenets of gradualism, and natural selection as the creative force of evolution.

Eminent evolutionist Ernst Mayr (2001) notes that one reason this controversy continues is because gradual transitions are not evident in the fossil record or even between living biota, but rather discontinuities are "overwhelmingly frequent." If evolution were gradual and continuous, one would expect to find transitions between taxa. Yet, there is no intermediary between whales and terrestrial mammals, nor between reptiles and mammals, nor reptiles and birds, nor flowering plants and their nearest relatives. Indeed, all phyla of animals are separated by a gap. Likewise, the fossil record shows striking discontinuities, with new species appearing suddenly. Evolutionists offer explanations for such phenomena, such as the incomplete sampling that results from the fossil record, but the very presence of such gaps is one reason for the controversy.

The second tenet inherent in the view that macroevolution is microevolution extended, the primacy of natural selection, has also been controversial since Darwin developed the theory. Concrete evidence for the theory of modification by natural selection is limited to microevolution, such as seen in the case of artificial selection, whereby various breeds of animals and varieties of plants have been produced that are different in some respect from their ancestors, or in the often-cited case of systematic color change in the peppered moth, Biston betularia, which was observed over a 50-year period in England. Microevolution can easily be demonstrated in the laboratory to the satisfaction of most observers. Large-scale changes, however, do not occur in directly observable time scales. The evidence that natural selection directs the major transitions between species and originates new designs is limited to extrapolation from these evidences on the microevolutionary level. This opens the possibility for other phenomena directing the evolutionary changes, such as species selection or even design by a supreme being.

In recent years, new models have been proposed that challenge the adequacy of gradualism and natural selection as models for macroevolution.

Punctuational models

The Darwinian and Neo-Darwinian emphasis on gradualism has been subject to re-examination on several fronts, including major trends in speciation and the origin of new designs.

Punctuated equilibrium

The orthodox view of evolution, which has dominated the field since the publication of The Origin of Species, is that new species are developed over a long period of time, by means of a gradual, steady, continuous process of transformation of the ancestral population. That is, small, microevolutionary changes accumulate over millions of years to create a new species or new design. In this model, the lack of a gradual series of intermediary forms, spread over a long time period, is attributed to the fact that the fossil record only provides an incomplete sampling of organisms. Historically, evolution of new taxa was viewed as requiring millions of years.

However, as noted by evolutionists Mayr (2001) and Gould (2000), the fossil record is quite extensive at the present time, and yet yields the same macroevolutonary pattern of species: Species tend to appear suddenly in the fossil record, and there are long periods where species do not change much (stasis). That is, one does not observe gradually graded sequences, but rather species tend to disappear from the fossil record looking much the same as when they first appeared.

The theory of punctuated equilibrium, largely crystallized and popularized by Eldredge and Gould (1972), ascribes that the fossil record accurately reflects evolutionary change. It maintains that macroevolutionary patterns of species are typically ones of morphological stability during their existence, and that most evolutionary change is concentrated in events of speciation—with the origin of a new species usually occurring during a geologically short periods of time when the long-term stasis of a population is punctuated by this rare and rapid event of speciation (Gould and Eldredge 1977). The sudden transitions between species may be measured on the order of hundreds or thousands of years relative to their millions of years of existence. Although the theory of punctuated equilibria originally generated a great deal of controversy, it is now viewed highly favorably in the scientific community and has even become a part of textbook orthodoxy.

The theory of punctuated equilibria has been embraced by many scientific creationists and intelligent design advocates as evidence that the fossil record does not support Darwinian theory. However, the founders and supporters of punctuated equilibrium emphasize their view that the pattern of punctuated equilibrium (stasis and rapid evolution) is the natural expectation from the now-generally accepted scientific model for speciation, involving evolution within peripherally isolated local populations.

What can be emphasized is that punctuated equilibrium merely addresses the pattern of evolution and is not tied to any one mode of speciation. Although occurring in a brief period of time, the species formation can go through all the stages, or can proceed by leaps. It is even agnostic with respect to natural selection. However, this theory has brought into acceptability a theistic view previously disparaged, that the fossil record supports the relatively sudden appearance of a species, and its morphological stability during its existence. Those who believe in a Supreme Being as creator can posit that it is that transcendental entity that directs the sudden changes.

Punctuated origin of new designs

Macroevolution "as microevolution extrapolated" has also been posited for the origin of new designs: such as the vertebrate eye, feathers, jaws in fish, vertebrates developing from invertebrates, and so forth.

To many observers, the development of such sophisticated new designs via such a chance process as natural selection seems problematic. However, evolutionary theory has dealt with such criticisms since the time of Darwin, offering two basic scenarios for how natural selection crafted such new designs.

  1. ) Complicated new designs have historically been explained as developing very gradually, involving numerous, tiny, imperceptible steps, with each step being advantageous and developed by natural selection. This style of argument follows Darwin's famous resolution proposed for the origin of the vertebrate eye.
  1. ) The origin of features that are not as easily visualized as along the lines postulated for the vertebrate eye are explained as involving pre-adaptations. One of Darwin's most cogent critics, St. George Mivart, argued that Darwinism cannot explain "the incipient stages of useful structures"—those structures that become useful only when they are fully formed. For example, how would bird feathers have evolved from reptilian scales? The partial transformation of a scale into a feather would seem to be useless for flight, so why would it have selective advantage? Similarly, what good is half a jaw in a fish? The principle of pre-adaptation, a gradualist approach, posits that the intermediary stages may perform useful functions different from the end stages, and thus have selective value. Partial feathers may have helped with heat regulation and incomplete jaws serve as a gill arch.

However, another solution to this macroevolution trend involves a discontinuity from microevolutionary processes. This solution posits that the full sequence of intermediate forms might not even have to exist, but that it is possible for major novelties to arise rapidly, discontinuously. Rather than view gill arch bones losing their connection to gills and migrating forward slowly over generations until a jaw is formed, perhaps a genetic change can result in a rapid change. Not only the lack of fossil evidence of intermediary forms promotes this view, but also the difficulty to even conceive of useful functional intermediates. Gould, for example, cites the fur-lined pouches of pocket gophers and the maxillary bone of the upper jaw of certain genera of boid snakes being split into front and rear halves:

How can a jawbone be half broken? … What good is an incipient groove or furrow on the outside? Did such hypothetical ancestors run about three-legged while holding a few scraps of food in an imperfect crease with their fourth leg?

The recent support among prominent evolutionists for the origin of major designs via rapid transitions aids theistic critiques countering gradual, natural selection as the creative force in evolution. Instead, it posits a scenario whereby natural selection could be seen as having only a secondary role—eliminating unfit organisms—rather than the main creative role. The diversity that natural selection acts on could have diverse sources, including even theistic origins. For such reasons, several prominent evolutionists have denounced the view of punctuational origins, and labeled such views non-Darwinian. Indeed, Darwin himself stated, immediately after his discussion of the evolution of the eye: "If it could be demonstrated that any complex organ existed, which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down."

However, the main proponents of punctuational origin are ardent evolutionists, such as Gould, who consider this theory to be within the Darwinian framework and, indeed, are careful to present the theory in a manner that supports the primacy of natural selection in evolution.

Species selection

The theory of "species selection" recognizes another model for macroevolutionary trends, and one that is not available on the microevolutionary level. Species selection holds that macroevolutionary trends result from the differential success of species. First proposed by Eldredge and Gould, and elaborated upon by Steven Stanley, this theory represents a fundamental break with the traditional Darwinian view that natural selection acts on the level of individual organisms. As Stanley (1975) states "macroevolution is decoupled from microevolution, and we must envision that process governing its course as being analogous to natural selection but operating at a higher level of organization." Gould (2002) observes "If organisms are the traditional units of selection in classical Darwinian microevolution within populations, then species operate in the same manner as basic units of macroevolutionary change."

Speciation events are viewed as occurring in all possible directions—a set of morphologies produced by speciation is essentially random with respect to the direction of evolutionary trends within a clade (Gould & Eldredge 1977). Species selection can result from either the differential rate of extinction or differential rate of speciation.

History of macroevolution

The debate over the relationship between macroevolution and microevolution has been going on since the 1860s, when evolution first became a widely accepted idea following the publication of Charles Darwin's The Origin of Species.

An early theory of macroevolution, Lamarckism, developed by biologist Jean-Baptiste Lamarck, asserted that individuals develop traits they use and lose traits they do not use, and that individuals pass the acquired traits onto their offspring. Lamarck asserted that when the environment changes, the "needs" of a species changed, resulting in individuals developing and passing on different traits, and finally leading to the transmutation of species. Lamarck's view was aligned with the concept of gradualism, but not natural selection.

Gregor Mendel, an Austrian monk, popularly known as the "father of modern genetics" for his discovery of the laws of genetic variation in his study of plants, believed that the laws of inheritance provided no grounds for macroevolution. In a lecture on March 8, 1865, Mendel noted that his research described the mechanism of microevolution, but gave no grounds for belief in macroevolution, saying:

No one will seriously maintain that in the open country the development of plants is ruled by other laws than in the garden bed. Here, as there, changes of type must take place if the conditions of life be altered, and the species possesses the capacity of fitting itself to its new environment. [However,] nothing justifies the assumption that the tendency to form varieties increases so extraordinarily that the species speedily lose all stability, and their offspring diverge into an endless series of extremely variable forms" (Henig 2000).

To the contrary, he said, the tendency is toward stability, with variation being the exception, not the rule.

Although Mendel's laws of inheritance were published as early as 1866, his theory was generally overlooked until the early twentieth century, in part because someone from outside the mainstream scientific community published it in an obscure journal. Darwin himself never read of Mendel's work, and his own proposed mechanism for inherited traits, pangenesis, was more useful for statisticians of the biometric school than it was for biologists. Darwin had discovered a variation ratio of 2.4:1 in a study of snapdragons that he published in 1868, similar to the 3:1 ratio that led Mendel to discover the laws of genetic variation. However, Darwin was not sure of its ultimate meaning (Henig 2000). After the rediscovery of Mendel's laws in 1900, there was some disagreement on its value as statisticians and biologists argued with each other, until they were reconciled by the work of R.A. Fisher in the 1930s.

Darwin himself saw no fundamental difference between microevolution and macroevolution. Likewise, this was the view of those involved in the modern evolutionary synthesis. In the late 1930s, evolutionary biologist Theodosius Dobzhansky helped devise the modern synthesis, or neo-Darwinism. His teacher was Russian entomologist Iurii Filipchenko (or Philipchenko), who coined the terms macroevolution and microevolution in his book Variabilitat und Variation, which included an early attempt to reconcile Mendelian genetics and evolution. Dobzhansky also used the terms. In bringing macroevolution and microevolution to the English language, he wrote "we are compelled at the present level of knowledge reluctantly to put a sign of equality between the mechanisms of macro- and microevolution" (Dobzhansky 1937). Some have argued that he was reluctant to equate macro- and microevolution because it went against the beliefs of his mentor, Filipchenko, who was an orthogenetist, and of the opinion that micro- and macroevolution were of a different mechanism and caliber (Burian 1994). From the writings of Dobzhansky, the modern synthesis view of evolution grew to its present prominence.

With the discovery of the structure of DNA and genes, genetic mutation gained acceptance as the mechanism of variance in the 1960s. The modern evolutionary synthesis, which remains prominent today, equated microevolution and macroevolution, asserting that the only difference between them was one of time and scale.

A few non-Darwinian evolutionists, however, including Schmalhausen and Waddington, argued that the processes of macroevolution are different from those of microevolution. According to these scientists, macroevolution occurs, but is restricted by such proposed mechanisms as developmental constraints. The concept can be summarized in Schmalhausen's Law, which holds:

When organisms are living within their normal range of environment, perturbations in the conditions of life and most genetic differences between individuals have little or no effect on their manifest physiology and development, but that under severe and unusual general stress conditions even small environmental and genetic differences have major effects.

Non-Darwinian evolution points to evidence of great changes in population under conditions of stress; however, the scientific community generally rejects it because it provides no mechanism for larger changes at a genetic level under those circumstances.

In the late 1970s, Stephen Jay Gould challenged the synthetic model of evolution, and proposed the punctuated equilibrium model, and other challenges to the status quo in evolutionary thinking. Gould stated,

I well remember how the synthetic theory [of evolution] beguiled me with its unifying power when I was a graduate student in the mid-1960s. Since then I have been watching it slowly unravel as a universal description of evolution… I have been reluctant to admit it—since beguiling is often forever—but if Mayr's characterization of the synthetic theory is accurate, then that theory, as a general proposition, is effectively dead, despite its persistence as textbook orthodoxy. (Gould 1980).

He further asserted, however, there is no doubt that descent with modification has happened, but that the debate is how it happened: "We are all trying to explain the same thing: the tree of evolutionary descent linking all organisms by ties of genealogy."

References

  • Burian, R. M. 1994. Dobzhansky on evolutionary dynamics: Some questions about his Russian background. In The Evolution of Theodosius Dobzhansky (Ed. M. B. Adams). Princeton, NJ: Princeton University Press.
  • Darwin, C. 1998 (1859). Origin of Species, New York: Modern Library.
  • Dobzhansky, T. 1937. Genetics and the Origin of Species. New York: Columbia University Press.
  • Henig, R. M. 2000. The Monk in the Garden: The Lost and Found Genius of Gregor Mendel, the Father of Genetics, Boston: Houghton Mifflin Company.
  • Gould, S. J. 2002. The Structure of Evolutionary Theory. Cambridge, MA: The Belknap Press of Harvard University Press.
  • Gould, S. J. 1980. Is a new and general theory of evolution emerging? Paleobiology 6: 119-130.
  • Luria, S. E., S. J. Gould, and S. Singer. 1981. A View of Life. Menlo Park, CA: The Benjamin/Cummings Publishing Company.
  • Mayr, E. 2001. What Evolution Is. New York: Basic Books.
  • Stanley, S. M. 1973. A theory of evolution above the species level. Proceedings of the National Academy of Sciences USA 72: 646-650.


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