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ences in reproductive fitness is known as natural selection. Gene flow between isolated populations slows down their genetic drift from each other and reduces the power of natural selection to promote divergence between them. When there is a great deal of gene flow between populations, they tend to be similar; in this way, gene flow has a homogenizing effect. The opposite also tends to be true: If there is little or no gene flow between populations, the genetic characteristics of each population are more likely to be different.

Gene flow does not just occur between two populations. When a series of populations exists over a large area, gene flow may serve to keep even the most distant populations similar to one another. This can occur even if they do not exchange individuals or gametes as long as the alleles from one population eventually flow into the other population through a series of migrations or gamete movements. Similarly, other types of separation can also be overcome by this type of graded gene exchange. For instance, Great Danes and Chihuahuas cannot breed directly because of size incompatibility. But gene flow in both directions, through intermediate-sized dogs, keeps these two breeds from becoming separate species.

It is very difficult to assess gene flow directly, so population geneticists have devised a way to estimate gene flow by comparing allele frequencies. By determining allele frequencies in two different populations, the amount of gene flow between them, usually expressed as the number of migrants exchanged per generation, can be estimated. see also Genetic Drift; Hardy-Weinberg Equilibrium; Population Genetics.

R. John Nelson

Bibliography

Avise, John C. Molecular Markers, Natural History and Evolution. New York: Chapman and Hall, 1994.

Futuyma, Douglas J. Evolutionary Biology, 3rd ed. Sunderland, MA: Sinauer Associates, 1998.

Mayr, Ernst. Evolution and the Diversity of Life: Selected Essays. Cambridge, MA: Belknap Press, 1976.

Weaver, Robert F., and Philip W. Hedrick. Genetics, 2nd ed. Dubuque, IA: William C. Brown, 1992.

Gene Targeting

Gene targeting is a method for modifying the structure of a specific gene without removing it from its natural environment in the chromosome in a living cell. This process involves the construction of a piece of DNA, known as a gene targeting vector, which is then introduced into the cell where it replaces or modifies the normal chromosomal gene through the process of homologous recombination.

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