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'''Microevolution''' refers to [[evolution]] that occurs at or below the level of [[species]], such as a change in the [[gene]] frequency of a population of organisms or the process by which new species are created ([[speciation]]). Microevolutionary changes may be due to several processes: [[mutation]], gene flow, genetic drift, and [[natural selection]].
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'''Microevolution''' refers to [[evolution]] that occurs at or below the level of [[species]], such as a change in the [[gene]] frequency of a population of organisms or the process by which new species are created ([[speciation]]). Microevolutionary changes may be due to several processes: [[mutation]], gene flow, genetic drift, and [[natural selection]].  
  
Biologists distinguish between microevolution and [[macroevolution]], the other main class of evolutionary phenomena. Macroevolution refers to evolution that occurs above the level of species, such as the origin of different phyla, the evolution of [[feather]]s, the development of vertebrates from invertebrates, and the explosion of new forms of life at the time of the [[Cambrian]] explosion.  
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Biologists distinguish between microevolution and [[macroevolution]], the other main class of evolutionary phenomena. Macroevolution refers to evolution that occurs above the level of species, such as the origin of different phyla, the evolution of [[feather]]s, the development of [[vertebrate]]s from [[invertebrate]]s, and the explosion of new forms of life at the time of the [[Cambrian]] explosion.  
  
 
However, microevolution also has been defined as only including evolutionary change below the level of species, not the process of speciation. When used in this manner, speciation is considered the purview of macroevolution.
 
However, microevolution also has been defined as only including evolutionary change below the level of species, not the process of speciation. When used in this manner, speciation is considered the purview of macroevolution.
  
Observable instances of evolution are examples of microevolution; for example, [[bacteria]]l strains that have become resistant to [[antibiotics]], or color changes in moths over time. Because microevolution can be observed directly, it is widely accepted, unlike macroevolution, which has engendered controversy since the time of [[Darwin]].  
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Observable instances of evolution are all examples of microevolution; for example, [[bacteria]]l strains that have become resistant to [[antibiotics]], or color changes in moths over time. Because microevolution can be observed directly, it is widely accepted, unlike macroevolution, which has engendered controversy since the time of [[Darwin]].  
  
[[Population genetics]] is the branch of biology that provides the mathematical structure for the study of the process of microevolution.
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[[Population genetics]] is the branch of biology that provides the mathematical structure for the study of the process of microevolution.  
  
==Overview==
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==Overview and evidences==
  
[[Evolution]] can be defined as .... gene change.   Microevolution is ....
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[[Evolution]] can be defined as any heritable change in a population of organisms over time, or, in terms of alleles (alternative forms of [[gene]]s), as any change in the frequency of alleles within a population. Both small changes, such as a slight increase in the numbers of antibiotic-resistant [[bacteria]] in a population of bacteria exposed to an [[antibiotic]], or large changes, such as the development of [[vertebrate]]s from [[invertebrate]]s, qualify as evolution.
  
Microevolution has been observed.....
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Microevolution refers to the small heritable changes that occur within a population or species.  
  
Notes:  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. The evidence that natural selection directs the major transitions between species and originates new designs (macroevolution) necessarily involves extrapolation from these evidences on the microevolutionary level. 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. 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. The validity of making this extrapolation has recently come under strong challenge from top evolutionists.
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Microevolution has been observed in both the laboratory and the field.
  
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===Laboratory evidences of microevolution===
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[[Image:Antibiotic_resistance.gif|thumb|size=180px|right| A change in the proportion of antibiotic-resistant bacteria in a population, after exposed to an antibiotic, is an example of microevolution.]]
  
====Punctuational models of speciation====
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In the laboratory, [[biology|biologists]] have demonstrated microevolution involving organisms with short lifecycles, such as [[fruit fly|fruit flies]], guppies, and [[bacteria]], which allow testing over many generations.  
Historically, the process of [[speciation]] has been viewed as involving the accumulation of small, microevolutionary changes in a population over time until a descendant [[species]] arises, either from the transformation of the ancestral population, or splitting from the ancestral population. Generally, the favored method for this was considered geographic isolation, such that a population becomes separate from the parental population, and develops into a new species by [[natural selection]] until reproductive isolation occurs, and there are two species. Reproductive isolation is therefore a secondary by-product of geographic isolation, with the process involving gradual allelic substitution.  
 
  
Contrasted with this view are recent punctuational models for speciation, whereby reproductive isolation arises rapidly, and not through gradual selection, but without selective significance (Gould 1980a; Gould and Eldredge 1977). That is, natural selection does not play a creative role in initiating speciation, nor in the definitive aspect of reproductive isolation, although it is usually postulated as the important factor in building subsequent adapation. One example of this is [[polyploidy]], where there is a multiplication of the number of chromosomes beyond the normal diploid number. Another models is chromosomal speciation, involving large changes in chromosomes due to various genetic accidents.
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Endler (1980) set up populations of guppies (''Poecilia reticulata'') and their predators in artificial ponds in the laboratory, with the ponds varying in terms of the coarseness of the bottom gravel. Guppies have diverse markings (spots) that are heritable variations and differ from individual to individual. Within 15 generations in this experimental setup, the guppy populations in the ponds had changed according to whether they were exposed to coarse gravel or fine gravel. The end result was that there was a greater proportion of organisms with those markings that allowed the guppies to better blend in with their particular environment, and presumably better avoid being seen and eaten by predators. When predators were removed from the experimental setup, the populations changed such that the spots on the guppies stood out more in their environment, likely to attract mates, in a case of [[Natural selection|sexual selection]].
  
One view, 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.
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Likewise, [[bacteria]] grown in a Petri dish can be given an antibiotic, such as [[penicillin]], that is just strong enough to destroy most, but not all, of the population. If repeated applications are used after each population returns to normal size, eventually a strain of bacteria with antibiotic resistance may be developed. This more recent population has a different allele frequency than the original population, as a result of selection for those bacteria that have a genetic makeup consistent with antibiotic resistance.
  
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.
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===Evidences in the field===
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In the field, microevolution has also been demonstrated. Both antibiotic-resistant bacteria and populations of pesticide-resistant [[insect]]s have been frequently observed in the field. In England, a systematic color change in the peppered moth, ''Biston betularia,'' has been observed over a 50-year period. While there is some controversy whether this later case can be attributed to [[natural selection]] (Wells 2000), the evidence of a change in the gene pool over time has been demonstrated. Since the introduction of house sparrows in [[North America]] in 1852, they have developed different characteristics in different locations, with larger-bodied populations in the north. This is assumed to be a heritable trait, with selection based on colder weather in the north.
  
==Microevolution versus macroevolution==
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A well-known example of microevolution in the field is the study done by Peter Grant and B. Rosemary Grant (2002) on Darwin's finches. They studied two populations of Darwin's finches on a Galapagos island and observed changes in body size and beak traits. For example, after a drought, they recorded that survivors had slightly larger beaks and body size. This is an example of an allele change in populations—microevolution. It is also an apparent example of natural selection, with natural selection defined according to [[Ernst Mayr|Mayr]] (2001) as: "the process by which in every generation individuals of lower fitness are removed from the population." However, the Grants also found an oscillating effect: when the rains returned, the body and beak sizes of the finches moved in the opposite direction.
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. "
 
  
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===Artificial selection===
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For thousands of years, humans have artificially manipulated changes within species through artificial selection. By selecting for preferred characteristics in cattle, horses, grains, and so forth, various breeds of [[animal]]s and varieties of [[plant]]s have been produced that are different in some respect from their ancestors. This also represents an example of microevolution, in that the changes coming from artificial selection are all within the level of the species.
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==Extrapolation of microevolution to macroevolution==
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The conventional view of [[evolution]] is that [[macroevolution]] is simply microevolution continued on a larger scale, over large expanses of time. That is, if one observes a change in the frequencies of spots in guppies within 15 generations, as a result of selective pressures applied by the experimenter in the laboratory, then over millions of years one can get [[amphibian]]s and [[reptile]]s evolving from fish due to [[natural selection]]. If a change in beak size of finches is seen in the wild in 30 years due to natural selection, then natural selection can result in new phyla if given eons of time.
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Indeed, the only concrete evidence for the [[Evolution#Theory_of_natural_selection|theory of modification by natural selection]]—that natural selection is the causal agent of both microevolutionary ''and'' macroevolutionary change— comes from microevolutionary evidences, which are then extrapolated to macroevolution. However, the validity of making this extrapolation has been challenged from the time of [[Darwin]], and remains controversial today, even among top evolutionists. Many see microevolution as decoupled from macroevolution in terms of mechanisms, with natural selection being incapable of being the creative force of macroevolutionary change. (See [[macroevolution]] and [[natural selection]].)
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==References==
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* Endler, J. A. 1980. Natural selection on color patterns in Poecilia reticulata. ''Evolution'' 34:76–91.
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* Endler, J. A. ''Natural Selection in the Wild.'' Princeton, NJ: Princeton University Press.
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* Grant, P. R. 1991. Natural selection and Darwin's finches. ''Scientific American'' 265:82–87.
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* Grant, P. R., and B. R. Grant. 1995. Microevolutionary responses to directional selection on heritable variation. ''Evolution'' 49:241–251.
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* Grant, P. R., and B. R. Grant. 2002. Unpredictable evolution in a 30-year study of Darwin's finches. ''Science'' 296(5568):707–711.
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* Mayr, E. 2001. ''What Evolution Is.'' New York: Basic Books.
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* Wells, J. 2000. ''Icons of Evolution.'' Washington, DC: Regnery.
  
  
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[[Category:Evolution]]

Latest revision as of 23:03, 3 April 2008


Microevolution refers to evolution that occurs at or below the level of species, such as a change in the gene frequency of a population of organisms or the process by which new species are created (speciation). Microevolutionary changes may be due to several processes: mutation, gene flow, genetic drift, and natural selection.

Biologists distinguish between microevolution and macroevolution, the other main class of evolutionary phenomena. Macroevolution refers to evolution that occurs above the level of species, such as the origin of different phyla, the evolution of feathers, the development of vertebrates from invertebrates, and the explosion of new forms of life at the time of the Cambrian explosion.

However, microevolution also has been defined as only including evolutionary change below the level of species, not the process of speciation. When used in this manner, speciation is considered the purview of macroevolution.

Observable instances of evolution are all examples of microevolution; for example, bacterial strains that have become resistant to antibiotics, or color changes in moths over time. Because microevolution can be observed directly, it is widely accepted, unlike macroevolution, which has engendered controversy since the time of Darwin.

Population genetics is the branch of biology that provides the mathematical structure for the study of the process of microevolution.

Overview and evidences

Evolution can be defined as any heritable change in a population of organisms over time, or, in terms of alleles (alternative forms of genes), as any change in the frequency of alleles within a population. Both small changes, such as a slight increase in the numbers of antibiotic-resistant bacteria in a population of bacteria exposed to an antibiotic, or large changes, such as the development of vertebrates from invertebrates, qualify as evolution.

Microevolution refers to the small heritable changes that occur within a population or species.

Microevolution has been observed in both the laboratory and the field.

Laboratory evidences of microevolution

A change in the proportion of antibiotic-resistant bacteria in a population, after exposed to an antibiotic, is an example of microevolution.

In the laboratory, biologists have demonstrated microevolution involving organisms with short lifecycles, such as fruit flies, guppies, and bacteria, which allow testing over many generations.

Endler (1980) set up populations of guppies (Poecilia reticulata) and their predators in artificial ponds in the laboratory, with the ponds varying in terms of the coarseness of the bottom gravel. Guppies have diverse markings (spots) that are heritable variations and differ from individual to individual. Within 15 generations in this experimental setup, the guppy populations in the ponds had changed according to whether they were exposed to coarse gravel or fine gravel. The end result was that there was a greater proportion of organisms with those markings that allowed the guppies to better blend in with their particular environment, and presumably better avoid being seen and eaten by predators. When predators were removed from the experimental setup, the populations changed such that the spots on the guppies stood out more in their environment, likely to attract mates, in a case of sexual selection.

Likewise, bacteria grown in a Petri dish can be given an antibiotic, such as penicillin, that is just strong enough to destroy most, but not all, of the population. If repeated applications are used after each population returns to normal size, eventually a strain of bacteria with antibiotic resistance may be developed. This more recent population has a different allele frequency than the original population, as a result of selection for those bacteria that have a genetic makeup consistent with antibiotic resistance.

Evidences in the field

In the field, microevolution has also been demonstrated. Both antibiotic-resistant bacteria and populations of pesticide-resistant insects have been frequently observed in the field. In England, a systematic color change in the peppered moth, Biston betularia, has been observed over a 50-year period. While there is some controversy whether this later case can be attributed to natural selection (Wells 2000), the evidence of a change in the gene pool over time has been demonstrated. Since the introduction of house sparrows in North America in 1852, they have developed different characteristics in different locations, with larger-bodied populations in the north. This is assumed to be a heritable trait, with selection based on colder weather in the north.

A well-known example of microevolution in the field is the study done by Peter Grant and B. Rosemary Grant (2002) on Darwin's finches. They studied two populations of Darwin's finches on a Galapagos island and observed changes in body size and beak traits. For example, after a drought, they recorded that survivors had slightly larger beaks and body size. This is an example of an allele change in populations—microevolution. It is also an apparent example of natural selection, with natural selection defined according to Mayr (2001) as: "the process by which in every generation individuals of lower fitness are removed from the population." However, the Grants also found an oscillating effect: when the rains returned, the body and beak sizes of the finches moved in the opposite direction.

Artificial selection

For thousands of years, humans have artificially manipulated changes within species through artificial selection. By selecting for preferred characteristics in cattle, horses, grains, and so forth, various breeds of animals and varieties of plants have been produced that are different in some respect from their ancestors. This also represents an example of microevolution, in that the changes coming from artificial selection are all within the level of the species.

Extrapolation of microevolution to macroevolution

The conventional view of evolution is that macroevolution is simply microevolution continued on a larger scale, over large expanses of time. That is, if one observes a change in the frequencies of spots in guppies within 15 generations, as a result of selective pressures applied by the experimenter in the laboratory, then over millions of years one can get amphibians and reptiles evolving from fish due to natural selection. If a change in beak size of finches is seen in the wild in 30 years due to natural selection, then natural selection can result in new phyla if given eons of time.

Indeed, the only concrete evidence for the theory of modification by natural selection—that natural selection is the causal agent of both microevolutionary and macroevolutionary change— comes from microevolutionary evidences, which are then extrapolated to macroevolution. However, the validity of making this extrapolation has been challenged from the time of Darwin, and remains controversial today, even among top evolutionists. Many see microevolution as decoupled from macroevolution in terms of mechanisms, with natural selection being incapable of being the creative force of macroevolutionary change. (See macroevolution and natural selection.)

References
ISBN links support NWE through referral fees

  • Endler, J. A. 1980. Natural selection on color patterns in Poecilia reticulata. Evolution 34:76–91.
  • Endler, J. A. Natural Selection in the Wild. Princeton, NJ: Princeton University Press.
  • Grant, P. R. 1991. Natural selection and Darwin's finches. Scientific American 265:82–87.
  • Grant, P. R., and B. R. Grant. 1995. Microevolutionary responses to directional selection on heritable variation. Evolution 49:241–251.
  • Grant, P. R., and B. R. Grant. 2002. Unpredictable evolution in a 30-year study of Darwin's finches. Science 296(5568):707–711.
  • Mayr, E. 2001. What Evolution Is. New York: Basic Books.
  • Wells, J. 2000. Icons of Evolution. Washington, DC: Regnery.


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