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Online Course for Teachers: Teaching Evolution

About this Course 

SESSION 4

SESSION 4: What Are the Processes for Evolution?

Explore Part E: Nonselective Mechanisms

Although natural selection is the major mechanism for evolution and speciation, there are several other mechanisms of evolution that are nonselective. These nonselective evolutionary mechanisms cause changes in populations because of chance fluctuations in the frequencies of genes. These chance fluctuations occur in large and small populations, but tend to be significant to evolution only in small populations.

Besides mutations, changes in the gene pool of populations can be caused by:

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gene flow: changes in the gene pool of a population because of the introduction of genes from another population by migration. Example: the gene pool in Southeast Asia was changed when U.S. soldiers had children with Vietnamese women during the Vietnam War in the 1960s and early 1970s.

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genetic drift: changes that take place because of random fluctuations of gene frequencies due to small population size (e.g., sampling size). Example: The frequency of Ellis-van Creveld syndrome -- a rare form of dwarfism that includes extra digits -- in the Amish population of eastern Pennsylvania, which has intermarried over many generations.
For more information, see:

Image of a six fingered hand.  Genetic Drift and Founder Effect

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founder effect: changes in a population when a small population moves to a new location bringing only a small fraction of the genetic variation of the parent population. The population then will contain only those genes the initial individuals brought with them. Example: The finches of the Galapagos Islands have the gene pool the original founding population of finches brought with them.

For more information on nonselective mechanisms in small populations see the Palomar College Web site.

Next: Explain Part A: Sexual Selection

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