Misplaced Pages

Revision as of 16:02, 24 August 2006

Macroevolution refers to evolution that occurs above the level of species, over long periods of time, that leads to speciation, in contrast to microevolution, which refers to smaller evolutionary changes (described as changes in allele frequencies) within a species or population. Within the Modern Synthesis school, microevolution is thought to be the only mode of evolution. The process of speciation (is isolated populations) can fall within the purview of either. Paleontology, evolutionary developmental biology, and comparative genomics contribute most of the evidence for the patterns and processes that can be classified as macroevolution.

Macroevolution is controversial in two ways:

• It is disputed among biologists whether there are macroevolutionary processes that are not described by strictly gradual phenotypic change, of the type studied by classical population genetics. One of these two views is becoming less and less tenable as the role for genome-wide changes and developmental processes in evolution become clearer.
• A misunderstanding about this biological controversy has allowed the concept of macroevolution to be coopted by creationists. They use this controversy as a supposed "hole" in the evidence for deep-time evolution.

However, microevolution and macroevolution both refer fundamentally to the same thing, changes in allele frequencies, and the scientific controversy is only about how those changes predominantly occur. Either way macroevolution uses the same mechanisms of change as those already observed in microevolution.

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 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

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. 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 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."

Creationism

Though the details of macroevolution are debated within the scientific community, it is widely accepted. 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 phenomenon, 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.

References

• 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.

External links

• Macroevolution as an independent discipline: Macroevolution in the 21st Century
• Macroevolution as the common descent of all life: "29+ Evidences for Macroevolution"
• From a creationist perspective: "A Critique of 29+ Evidences for Macroevolution"
• A reply to the critique of "29+ Evidences"

• Evolutionary biology