Microevolution is the occurrence of small-scale changes in allele frequencies in a population, over a few generations, also known as change at or below the species level [1].
These changes may be due to several processes: mutation, natural selection, gene flow, genetic drift and nonrandom mating.
Population genetics is the branch of biology that provides the mathematical structure for the study of the process of microevolution. Ecological genetics concerns itself with observing microevolution in the wild. Typically, observable instances of evolution are examples of microevolution; for example, bacterial strains that have antibiotic resistance.
Microevolution can be contrasted with macroevolution; which is the occurrence of large-scale changes in gene frequencies, in a population, over a geological time period (i.e. consisting of extended microevolution). The difference is largely one of approach. Microevolution is reductionist, but macroevolution is holistic. Each approach offers different insights into the evolution process.
[edit] Origin of the term
Russian Entomologist Yuri Filipchenko (or Philipchenko, depending on the transliteration) first coined the terms "macroevolution" and "microevolution" in 1927 in his German language work, "Variabilität und Variation". The term was brought into English-speaking by Theodosius Dobzhansky in his book Genetics and the Origin of Species (1937)[1].
Since the inception of the two two terms, their meanings have been revised several times and even fallen into disfavor amongst scientists who prefer to speak of biological evolution as one process[1]. The term was returned, somewhat, to prominence in the last thirty years due to breakthroughs in evolutionary theory that seem to indicate that there are different processes involved in speciation than simple modification.[citation needed]
Macroevolution refers to evolution that occurs at or above the level of species, in contrast with microevolution,[citation 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.
Within the Modern Synthesis school of thought, macroevolution is thought of as the compounded effects of microevolution. Thus, the distinction between micro- and macroevolution is not a fundamental one - the only difference between them is of time and scale. This understanding is disputed by some biologists, who claim that there may be macroevolutionary processes that cannot be described by strictly gradual phenotypic change, of the type studied by classical population genetics.
Some creationists have also adopted the term "macroevolution" to describe the form of evolution that they reject. They may accept that evolutionary change is possible within species ("microevolution"), but deny that one species can evolve into another ("macroevolution"). These arguments are rejected by mainstream science, which holds that there is ample evidence that macroevolution has occurred in the past.[1][2]
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] Origin of the Term
Russian Entomologist Yuri Filipchenko (or Philipchenko), depending on the transliteration) first coined the terms "macroevolution" and "microevolution" in 1927 in his German language work, "Variabilität und Variation"[1].
Since the inception of the two two terms, their meanings have been revised several times and even fallen into disfavour amongst scientists who prefer to speak of biological evolution as one process[1]. The term was returned, somewhat, to prominence in the last thirty years due to breakthroughs in evolutionary theory that seem to indicate that there are different processes involved in speciation than simple modification.
[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, a Moravian[3] 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."[4]. 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[5] observes that in 1940, 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.”[6]
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 Ivan Ivanovich Schmalhausen and Conrad Hal 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[1].
[edit] Punctuated equilibria
In the late 1970s, 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-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." (Paleobiology, Vol.6, 1980, p. 120). He also was not proposing additional mechanisms to the modern synthesis but was only maintaining that life's history is not in a constant state of gradual evolving (The Structure of Evolutionary Theory pp 1006-1021)
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."
2007-05-05 19:22:07
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answer #7
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answered by wierdos!!! 4
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