Microevolution
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.)
ReferencesISBN 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.
