Difference between revisions of "Microevolution" - New World Encyclopedia

From New World Encyclopedia
Line 26: Line 26:
 
==Extraopolation of microevolution to macroevolution==
 
==Extraopolation of microevolution to macroevolution==
  
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.
+
The convention view of evolution is that [[macroevolution]] is simply microevolution continued over large expanses of time. That is, if one sees guppies changing the frequencies of their markings in 15 generations in the laboratory, then over millions of years one can see [[amphibian]]s and [[reptile]]s evolving from fish. If a change in beak size of finches is seen in the wild in 30 years, then new phyla can arise given eons of time.
 
 
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. "
 
 
 
 
 
 
 
====Punctuational models of speciation====
 
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.
 
 
 
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.
 
 
 
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.
 
  
 +
Indeed, the only concrete evidence for the [[Evolution#Theory_of_natural_selection|theory of modification by natural selection]] 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 macroevoluton in terms of mechanisms, with natural selection being incapable of being the creative force of macroevolutionary change. (See [[macroevolution]] and [[natural selection]].)
  
 
==References==
 
==References==

Revision as of 18:27, 12 June 2006


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 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

Evolution can be defined as any heritiable 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-resistent 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 slight, heritable changes within a population or species. It has been observed in both the laboratory and the field.

In the laboratory, biologists have demonstrated microevolution on organisms with short life-cycles, such as fruit flies and guppies, 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. Within 15 generations, the guppy populations had changed such that they had greater proportions of those markings (spots, which were heritable variations) that allowed the guppies to better blend in with their environment. When predators were removed, the populations changes such that the spots on the guppies stood out more, assumedly to attract mates.

Of course, for thousands of years, humans have artificially manipulated changes within species through artificial selection, by selecting for preferred characteristics in cows, horses, grains, and so forth. As a result, various breeds of animals and varieties of plants have been produced that are different in some respect from their ancestors.

In the field, microevolution has also 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. The development of antibiotic resistent bacteria and pesticide-resistent insects has been frequently observed in the field, and 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 allele changes over time has been demonstrated. Another 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 two beak traits.


Extraopolation of microevolution to macroevolution

The convention view of evolution is that macroevolution is simply microevolution continued over large expanses of time. That is, if one sees guppies changing the frequencies of their markings in 15 generations in the laboratory, then over millions of years one can see amphibians and reptiles evolving from fish. If a change in beak size of finches is seen in the wild in 30 years, then new phyla can arise given eons of time.

Indeed, the only concrete evidence for the theory of modification by natural selection 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 macroevoluton 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., 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, NY: Basic Books.
  • Wells, J. 2000. Icons of Evolution. Washington, D.C.: Regnery.


Credits

New World Encyclopedia writers and editors rewrote and completed the Wikipedia article in accordance with New World Encyclopedia standards. This article abides by terms of the Creative Commons CC-by-sa 3.0 License (CC-by-sa), which may be used and disseminated with proper attribution. Credit is due under the terms of this license that can reference both the New World Encyclopedia contributors and the selfless volunteer contributors of the Wikimedia Foundation. To cite this article click here for a list of acceptable citing formats.The history of earlier contributions by wikipedians is accessible to researchers here:

The history of this article since it was imported to New World Encyclopedia:

Note: Some restrictions may apply to use of individual images which are separately licensed.