Microevolution

From New World Encyclopedia


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 and evidences

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.

Laboratory evidences of microevolution

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. Guppies have markings (spots) which are heritable variations. Within 15 generations, the guppy populations had changed such that they had greater proportions of those markings that allowed the guppies to better blend in with their environment, and presumedly better avoid being seen and eaten by predators. When predators were removed, the populations changed such that the spots on the guppies stood out more, assumedly to attract mates, in a case of sexual selection.

Likewise, bacteria grown in a petri dish can be given just strong enough of an antibiotic, such as penicillin, 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 because of selection for those bacteria that have a genetic makeup consistent with antibiotic resistence.

Evidences in the field

In the field, microevolution has also been demonstrated. Both antibiotic resistent bacteria and populations of pesticide-resistent 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 allele changes 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 two 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 as Mayr (2001) does: "the process by which in every generation individuals of lower fitness are removed from the population." However, the Grant's also found an occilating 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 electing for preferred characteristics in cows, 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.

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
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  • 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.


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