Microevolution

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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 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.

Contents

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

  • 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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