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Limited Penetration of Barriers as a Factor in Evolution

D. B. O. Savile
Evolution
Vol. 13, No. 3 (Sep., 1959), pp. 333-343
DOI: 10.2307/2406110
Stable URL: http://www.jstor.org/stable/2406110
Page Count: 11
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Limited Penetration of Barriers as a Factor in Evolution
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Abstract

If two adjacent land masses are effectively separated for prolonged periods each acquires a relatively stable biota, but the two biotas will be appreciably different. The organisms in each will be subject to strong and constant competition and all will be highly adapted to the existing conditions. Consequently almost all mutations will be deleterious and evolution will be restricted. If an organism manages to cross the barrier to the other continent, and to survive, it will no longer be in balance with the community, some of its mutations will be newly adaptive, and it may radiate vigorously. Some of the radiants may return across the barrier and radiate again. Consequently it may be impossible to deduce the area in which a group originated. A somewhat similar process may operate when ecological barriers are penetrated. If many organisms cross a barrier nearly simultaneously the competition remains more intense and the chance of any one of them radiating strongly is reduced. Organisms in an isolated continent are not subject to selection for resistance to diseases that occur in other continents and are frequently highly susceptible to them. Occasional intercontinental connections may result in the introduction of exotic diseases and of tolerant or resistant organisms to carry them, which may explain some of the abrupt extinctions recorded by the paleontologist.

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