Macroevolution

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Macroevolution refers to evolution that occurs at or above the level of species, in contrast with microevolution^{[attribution needed]}, which refers to smaller evolutionary changes (typically described as changes in allele frequencies) within a species or population. The process of speciation may fall within the purview of either, depending on the forces thought to drive it. Paleontology, evolutionary developmental biology, and comparative genomics contribute most of the evidence for the patterns and processes that can be classified as macroevolution. An example of macroevolution is the appearance of feathers during the evolution of birds from one group of dinosaurs.

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. Within the Modern Synthesis school of thought, microevolution is thought to be the only mode of evolution (i.e. what is sometimes thought of as "macroevolution" actually consists of the compounded effects of microevolution - the only difference between them is one of time and scale).
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.[2] [3]

1 Research topics
2 History of macroevolution
3 Criticisms of macroevolution
4 See also
5 References
6 External links

[edit] 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

[edit] 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, this 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.

[edit] Modern evolutionary synthesis

In the late 1930s, evolutionary biologist Theodosius Dobzhansky devised the Modern evolutionary synthesis. 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."^[1]. 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.

Welsey Elsberry ^[2] observes that in 1940, ^[3] Richard Goldschmidt “proposed that macroevolution refer to the establishment of the good species and higher taxa. Microevolution would then refer to everything below the species level.”

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.

[edit] Non-Darwinian evolutionists

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.

[edit] Punctuated equilibria

In the late 1970's, Stephen Jay 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."

[edit] Criticisms of macroevolution

While details of macroevolution are continuously studied by the scientific community, the overall theory behind macroevolution (i.e. common descent) has been overwhelmingly consistent with empirical data. Predictions of empirical data from the theory of common descent have been so consistent that biologists often refer to it as the "fact of evolution" (Theobald 2004). Nevertheless, macroevolution is sometimes disputed by religious groups. Generally speaking, these groups attempt to differentiate between microevolution and macroevolution, asserting various hypotheses which are considered to have no scientific basis by any mainstream scientific organization, including the American Association for the Advancement of Science^[4].

Much of the debate encircling the validity of macroevolution as a distinct evolutionary process involves primarily two factors: (1) species stasis (a pattern in which species show no net morphological change over millions of years) and (2) species selection (selection at the species level where the individuals experiencing differential reproduction or death are species rather than organisms as is typically the case in Neo-Darwinism). Since the discovery of Punctuated Equilibria in the fossil record, it has often been questioned whether these two processes (species stasis and species sorting) require deviation from typical explanations which adhere to Darwinian (or Synthetic) orthodoxy. Recent work on stasis by Eldredge and others has shown, however, that stasis is often the byproduct of species being broken up into several, quasi-autonomous lineages. Clearly, if these lineages (presumably discrete populations) are sufficiently autonomous, then the entire species cannot exhibit morphological change because it does not evolve as a coherent unit. In such instances where each of the populations is effectively evolutionary independent, it would be virtually impossible for the entire species to exhibit net morphological change in the absence of rampant parallelism (a very unlikely proposition since this invokes an almost orthogenetic or teleological perspective).

Species selection, on the other hand, has been a more difficult problem to evaluate empirically because it is a trickier concept to pin down. While Coyne and Orr recently discussed numerous potential and empirical cases of species selection, there has been no consensus over a working definition of species selection in spite of more than 30 years of debate. Some camps (Vrba and Lieberman) insist that selection requires emergent characters while others (Gould and Lloyd) contend that only emergent fitness differences are required for the operation of selection processes. Since Darwin chose not to differentiate between "emergent" or "reducible" characters when describing male adaptations resulting from asymmetrical polyandry where no female behaviors or preferences for such macroscopic traits were evident (a phenomenon now know as "sperm competition"), historical posterity clearly defers to Gould and Lloyd's conception that only fitness differences are important for selection processes. Also, it makes little sense to distinguish between this form of "male-male" competitive sexual selection and other types involving allegedly emergent characters, such as male antlers. Clearly, both male gametocytes and male antlers are properties of males, and to say that these are fundamentally different types of selection based on whether one trait is macroscopic while the other is microscopic (appearing only on individual sex cells) is not the conventional approach. As discussed in greater detail above, Darwin treated selection as an economic process involving evolutionary assets of individuals, regardless of whether these traits were macroscopic vs. microscopic. This conceptual distinction is a recent invention, and so it is likely that Coyne and Orr are correct in asserting that there is strong empirical evidence for the operation of species selection in nature with well over a hundred sister groups analyzed thus far.

[edit] See also

[edit] References

^ Dobzhansky, Theodosius Grigorievich (1937). Genetics and the origin of species. LC QH366 .D6. , p12
^ [1]
^ Goldschmidt, Richard (1940). The material basis of evolution. New Haven, Yale University Press. LC QH366 .G53.
^ AAAS press release news-links and resources

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.
Theobald, Douglas L. '29+ Evidences for Macroevolution: The Scientific Case for Common Descent.' The Talk.Origins Archive. Vers. 2.83. 2004. 12 Jan, 2004 <http://www.talkorigins.org/faqs/comdesc/>