Geographic Isolation

How do two populations of organisms that initially have the same gene pool come to have gene pools different enough to prevent their interbreeding? Geographic or other isolation of the two populations from each other prevents the flow of genes between the two populations, and random mutations, which are rarely identical, then spread only throughout the population in which they arise. Imagine, for example, that a population of white daisies occurs throughout a wide valley that is bounded on both sides by high mountains. If a mutation occurs that results in the white part of a daisy becoming red, the new color may, in time, if other environmental conditions permit, spread throughout the entire population. If, however, before the mutation occurs, there is a volcanic eruption that blocks off part of the valley, we would then have two populations of white daisies isolated from one another. If the same mutation for red color should then occur, it would spread only through one population. Meanwhile, if a mutation for hairy leaves occurs in the other population, that characteristic could spread throughout the second population but be prevented by the geographic barrier from spreading to the population of red daisies. In time, the genetic changes may become so great that even if the isolation is removed, gene flow between the two populations no longer can occur, and two distinct species of daisies would be the result.

In the United States, there are two closely related species of small trees or shrubs called redbuds (Cercis) that look very much alike. The eastern redbud (Cercis canaden-sis) occurs on the borders of streams, mostly east of the Mississippi River between the Canadian border and Florida, where some form of precipitation occurs throughout the year. The western redbud (Cercis occidentalis) is native to stream areas in California, Utah, Nevada, and Arizona, where most of the precipitation occurs in the winter and spring. The two species can be artificially hybridized, but

Chapter 15

Chapter 15

Geographic Isolation

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