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Species - Speciation

allopatric population isolated reproductively

Speciation is the process whereby a single species develops over time into two distinct reproductively isolated species. Speciation events are of two types—either allopatric or sympatric. Allopatric speciation results from the division of a population of organisms by a geographical barrier. The isolation of each of the two populations slowly results in differences in the gene pools until the two populations are unable to interbreed either because of changes in mating behavior or because of incompatibility of the DNA from the two populations. The early stages of allopatric speciation are often evident when one examines the same species of fish from different ponds. Fish from the two ponds may not appear to be morphologically different, but there may be slight differences in the gene pools of each population. If the two fish populations remain separated for enough generations, they may eventually become two separate reproductively isolated species.

Sympatric speciation is less frequent than allopatric speciation and occurs when a group of individuals becomes reproductively isolated from the larger population occupying the same range. This type of speciation may result from genetic changes (or mutations) occurring within individuals that inhibits them from interbreeding with others, except those in which the same mutation has occurred. Polyploid plant species, that is, species with more than two copies of each chromosome, are thought to have arisen by sympatric speciation.

More than 1.5 million species have been described and it is estimated that there are between 10-50 million species currently inhabiting Earth.

See also Genetics; Mutation; Taxonomy.


Resources

Books

Campbell, N., J. Reece, and L. Mitchell. Biology. 5th ed. Menlo Park: Benjamin Cummings, Inc. 2000.

Cockburn, Andrew. An Introduction to Evolutionary Ecology. Boston: Blackwell Scientific Publications, 1991.

Mayr, Ernst, and Peter Ashlock. Principles of Systematic Zoology. 2nd ed. New York: McGraw-Hill, 1991.

Wilson, Edward. The Diversity of Life. Cambridge, Massachusetts: Belknap Press, 1992.


Steven MacKenzie

KEY TERMS

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Allopatric speciation

—Speciation resulting from a population being geographically divided.

Linnean system

—Classification scheme used by taxonomists which places organisms into a hierarchy of groups.

Morphology

—The physical properties possessed by an organism.

Polyploid

—An organism with more than two copies of each chromosome.

Sympatric speciation

—Speciation that occurs when a subpopulation becomes reproductively isolated from a larger population occupying the same range.

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