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.
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.
However, since the time of Darwin, the concept of macroevolution has also been controversial. Many evolutionists see macroevolution as simply a continuation of microevolution on a great scale, while others see it as disconnnected from microevolution. Many religous adherents *
Research Topics
Some examples of subjects whose study falls within the realm of macroevolution:
- The debate between punctuated equilibrium and gradualism
- Speciation and extinction rates
- Mass extinctions
- Adaptive radiations such as the The Cambrian Explosion
- Changes in biodiversity through time
- The role of development in shaping evolution, particularly such topics as heterochrony and developmental plasticity
- Genomic evolution, like horizontal gene transfer, genome fusions in endosymbioses, and adaptive changes in genome size
Overview
There are two view of macroevolution: (1) that it is simply an extention of microevolutionary processes over large time scales, and (2) that it is disconnected from microevolution, or involves other processes, such as punctuational change.
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). Gradulism is a view of evolution as proceeding by means of 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 transistions, 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.
Since the time of Darwin, there has been controversy over whether macroevolution is simply an extrapolation or continuation of microevolution, or may be considered a separate phenomena, involving different theories. Even today, the validity of making this extrapolation has come under strong challenge from top evolutionists, because of views on these two central tenets of gradualism, and natural selection as the creative force.
Eminent evolutionist Ernst Mayr (2001) notes that one reason this controversy continues is because gradual change is 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 transitions between taxa. Yet, there is no intermediary between whales and terrestrial mammals, nor between reptiles and mammals, or reptiles and birds, or flowering plans and their nearest reptiles. Indeed, all phyla of animals are separated by a gap. Likewise, the fossil record shows striking dinscontinuities, 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 of 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, but somewhat problematic 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 direclty 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 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
This Darwinian and Neo-Darwinian emphasis on gradualism has been subject to re-examination on several levels: the levels of speciation, origin of new designs, and major evolutionary trends.
Punctuational speciation
A common misconception about evolution is that the development of new species requires millions of years. Indeed, the gradualist view that speciation involved a slow, steady, progressive transformation of an ancestral population into a new species has dominated much of evolutionary thought from the time of Darwin. Such a transformation was generally viewed as involving large numbers of individuals, being even and slow, and occurring over all or a large part of the ancestral species geographic range. The absence of a gradually graded sequence of intermediary forms in the fossil record was attributed to the imperfection of the geological record. However, the fossil record is considerably more complete than it was at the time of Darwin, and it still yields the same two points: (1) the sudden appearance of species; and (2) long periods where species do not change much. Indeed, the principle feature of individual species within the fossil record is that they do not change. Species first appear in the fossil record looking much the same as when they disappear. One observes a sudden appearance of fully formed species in the geological record. The theory of punctuated equilibria ascribes that the fossil record accurately reflects evolutionary change. That is, it posits 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 geologically short periods of time when the long-term stasis of a population is punctuated by this rare and rapid event of speciation. The sudden transitions between species are sometimes measured on the order of 100s or 1000s of years relative to their millions of years of existence. Although the theory of punctuated equilibria originally generated a lot of controversy, it is now viewed highly favorably in the scientific community, and has even become a part of recent textbook orthodoxy.
The theory of punctuated equilibria has been embraced by many scientific creationists as evidence that the fossil record does not support Darwinian theory. However, the founders and supporters of punctuated equilibria emphasize their view that the pattern of punctuated equilibria (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 equilibria 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 creator Supreme Being can posit that it is God who directs the sudden changes. I would like to add that I first was exposed to the essential ideas of the theory of punctuated equilibrium – stasis and step-wise evolution – not from the theory’s founders, Stephen Gould and Niles Eldridge, but independently from a religious leader, and based on religious rather than scientific principles. It was through an early 1970’s publication by Dr. Sang Han Lee, a Unificationist scholar, who asserted that the teaching of the Rev. Dr. Sun Myung Moon on evolution lead to the prediction that evolutionary change would have to be step-wise. That is, if the fossil record were complete, it would have to show that each species remains virtually the same throughout its existence, and then there would be a sudden change or splitting to create a new species from the existing species. I ran across these writings in the late 1970s, at which time, this presentation on evolutionary theory ran counter with the overwhelming, prevailing orthodoxy of evolutionary theory. However, this view is near identical with this new orthodoxy of punctuated equilibria.
Punctuated origin of new designs
There is also the issue of the origin of new designs: such as the vertebrate eye, feathers, or jaws in fish. Such issues have often been used by critics to counter Darwinian theory. To most observers, the development of such sophisticated new designs via such a random process as natural selection seems inconceivable. However, evolutionary theory has dealt with such criticisms since the time of Darwin, offering three 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. (2) The origin of many other features are not as easily explained along the lines postulated for the vertebrate eye. For example, Darwin's most cogent critic, St. George Mivart, argued that Darwinism must fail because it cannot explain "the incipient stages of useful structures" — those structures that become useful only when they are fully formed. For example, bird feathers evolved from reptilian scales. If they are for flight, what possible benefit could they have conferred in their early stages? A scale transformed 5% of the way into a feather would be useless in flight; so, how could such an "incipient stage" arise by natural selection? What about the jaws of fishes? What good is half a jaw? For such origins as feathers and jaws, this thorny issue is generally resolved by evolutionists using the principle of preadaptation, a gradualist approach. Preadaptation holds that intermediate stages in the development of major evolutionary novelties often perform functions different from those of final stages. By such explanations, a gradual transition can be proposed for structures which cannot function in a certain way until they are fully formed. In other words, various structures functioned in one role for ancestors, but by good fortune prove well suited after transformation to perform a very different role for descendants. Thus, feathers may have served originally for heat regulation or catching prey, and only late in development were converted to usage in flight. Likewise, the bony support for jaws may have originally served as a gill arch. (3) However, another solution for origin of new designs, which is gaining renewed attention among evolutionists, is that the full sequence of intermediate forms need not have existed at all, and instead some major novelties may have arisen rapidly, discontinuously. This in not to suggest that the first complete bird hatched from a fully reptilian egg or that a jawed fish arose all at once, fully formed. However, this view does question, in the evolution of the jaw, for example, whether one can really believe that the front set of gill arch bones lost their connection to the gills and migrated slowly forward, a fraction of a millimeter per generation, until they surrounded the mouth and took on their new function. Instead, it questions, is it not more likely that a genetic change resulted in the transition as a kind of switch: the bones are either back as gill supports or forward as mouth support? This view of a punctuational origin of key features arose because of: (1) the persistent problem of the lack of fossil evidence for intermediate stages between major designs, with transitions between major groups being characteristically abrupt; and (2) the inability, even in one's imagination, to even construct functional intermediates in many cases. Prominent evolutionist Stephen Jay 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. For one, such a punctuational model recognizes the lack of intermediates in the fossil record and advances the difficulty of even imagining such intermediates. It also posits a scenario whereby natural selection could be seen as having only a secondary role — eliminating unfit organisms — rather than the main creative role. For such reasons, several prominent evolutionists have denounced the view of punctuational origins, and labeled such views non-Darwinian.
Indeed, Darwin himself had 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." It should be noted, however, that the main proponents of punctuational origin are ardent evolutionists, 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.
This theory, punctuated equilibrium, put forth by Stephen Jay Gould, is based on the fact that there are critical genes (such as the homeobox) in all living organisms, and a small change in them could cause drastic changes in the organism, resulting in a new species quite rapidly.
Single small mutations are sometimes the main difference between one species and another. Scientists have discovered very important genes, such as the homeobox, which regulate the growth of animals in their embryonic state. Scientists have managed to create new species of fly by irradiating the homeobox gene, causing a radical mutation in the development of the segments of the body. The fly may grow an extra thorax, or grow legs out of its eyestalks, all due to a single base pair alteration. The additional information needed for these structures did not arise from the mutation, of course, but existed elsewhere in the animal's DNA and was replicated at the novel location. It has been proposed that centipedes and millipedes originated from insect precursors, but their homeobox gene mutated and they ended up growing dozens of body segments instead of just one. A very small change, and an entire species is formed.
It must be noted that many mutations are common and unexpressed, particularly when it involves toggling of the third base sequence in a codon. Most deleterious mutations are not seen simply because they do not result in viable reproduction.
It is argued that, since macroevolution can not be confirmed by a controlled experiment, it cannot be considered to be part of a scientific theory. However, evolutionists counter that astronomy, geology, archaeology and the other historical sciences, like macroevolution, have to check hypotheses through natural experiments. They confirm hypotheses by finding out if they conform or fit with the physical or observational evidence and can make valid predictions. In this way, macroevolution is testable and falsifiable.
Most scientists consider large gaps between taxonomic groups to be explainable by ecological/evolutionary factors, such as extinctions, population bottlenecks, and the emergence of unoccupied ecological niches.
Speciation
Mutations of the homeobox and other critical genes are sometimes called macromutations, which cause the addition of body segments among the Arthropoda. One major problem lies in the scales of resolution offered by biological techniques. The fossil record cannot record events that happened in less than a million years, which allows it to clearly show slow speciation events that are the result of accumulated mutations over a long time, but records sudden "jumps" in species that are most likely the result of mutations in the critical regulatory genes in only a few generations. Macromutations are probably the best explanation of the Cambrian Explosion that occurred 550 million years ago.
Creationism
Though the details of macroevolution are debated within the scientific community, it is widely accepted. But it has become increasingly controversial in the public arena. Macroevolution is largely disputed by many creationism and intelligent design advocates. Generally speaking, these groups differentiate between microevolution and macroevolution, asserting that the former is an undeniably observable phenomena, but that the latter is not. They have proposed a number of limits beyond which they assert evolution cannot occur. Proponents of Intelligent design argue that the mechanisms of evolution are incapable of giving rise to instances of specified complexity and irreducible complexity. Proponents of creation biology assert that life was originally created in a finite number of discrete created kinds beyond which and between which no evolution can occur. The argument being that the dominant source of biological change is population isolation, genetic drift and mutation which causes the loss of the diversity of the original kinds and genetic information, rather than an increase of genetic diversity through mutation or other evolutionary mechanisms.
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.
The first 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 environmental changes changed the "needs" of a species, which caused it to develop different traits, leading to the transmutation of species.
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 natural variation in 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." To the contrary, he said, the tendency is toward stability, with variation being the exception, not the rule. (Henig, 141)
Darwin, on the other hand, saw no fundamental difference between microevolution and macroevolution. He asserted that "Certainly no clear line of demarcation has as yet been drawn between species and sub-species — that is, the forms which in the opinion of some naturalists come very near to, but do not quite arrive at, the rank of species: or, again, between subspecies and well-marked varieties, or between lesser varieties and individual differences. These differences blend into each other by an insensible series; and a series impresses the mind with the idea of an actual passage." (Darwin, 77)
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 it was published in an obscure journal and by someone from outside the mainstream scientific community. 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 which 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, 143) After the rediscovery of Mendel's laws in 1900, the statisticians and biologists argued against each other until they were reconciled by the work of R.A. Fisher in the 1930s.
In the late 1930s, evolutionary biologist Theodosius Dobzhansky devised the Modern evolutionary synthesis. In bringing macroevolution and microevolution to the English language, 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, 12). 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 calibre. (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. This developing theory of evolution was then called the modern evolutionary synthesis, which remains prominent today. The synthetic model of evolution equated microevolution and macroevolution, asserting that the only difference between them was one of time and scale.
A few non-Darwinian evolutionists remained, however, including Schmalhausen and Waddington, who 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 that "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, it is generally rejected by the scientific community because it provides no mechanism for larger changes at a genetic level under those circumstances. For a discussion of Schmalhausen's theory of "canalization," see this article.
In the late 1970's, Stephen J. Gould challenged the synthesis model of evolution, and proposed a punctuated equilibrium model, whereby major evolutionary changes took place in limited gene pools after radical climate changes. He said, "I well remember how the synthetic theory [of evolution] beguiled me with its unifying power when I was a graduate student in the mid-1960's. 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." (Paleobiology, Vol.6, 1980, p. 120).
Despite his rejection of the synthetic theory, however, he asserted that "Evolutionary theory is now enjoying this uncommon vigor. Yet amidst all this turmoil no biologist has been led to doubt the fact that evolution occurred; we are debating how it happened. We are all trying to explain the same thing: the tree of evolutionary descent linking all organisms by ties of genealogy. Creationists pervert and caricature this debate by conveniently neglecting the common conviction that underlies it, and by falsely suggesting that evolutionists now doubt the very phenomenon we are struggling to understand."
ReferencesISBN links support NWE through referral fees
- Darwin, Origin of Species, New York: Modern Library, 1998.
- Dobzhansky, Th: 1937. Genetics and the Origin of Species, Columbia University Press
- Henig, The Monk in the Garden: The Lost and Found Genius of Gregor Mendel, the Father of Genetics, Boston: Houghton Mifflin Company, 2000.
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