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California Standards Biology

Standard 8d Preknowledge

8d) Students know reproductive or geographic isolation affects speciation.


Events that lead to reproductive isolation of populations of the same species cause new species to appear. Barriers to reproduction that prevent mating between populations are called prezygotic (before fertilization) if they involve such factors as the isolation of habitats, a difference in breeding season or mating behavior, or an incompatibility of genitalia or gametes. Postzygotic (after fertilization) barriers that prevent the development of viable, fertile hybrids exist because of genetic incompatibility between the populations, hybrid sterility, and hybrid breakdown.

These isolation events can occur within the geographic range of a parent population (sympatric speciation) or through the geographic isolation of a small population from its parent population (allopatric speciation). Sympatric speciation is much more common in plants than in animals. Extra sets of chromosomes, or polypoidy, that result from mistakes in cell division produce plants still capable of long-term reproduction but animals that are incapable of that process because polypoidy interferes with sex determination and because animals, unlike most plants, are usually of one sex or the other. Allopatric speciation occurs in animal evolution when geographically
isolated populations adapt to different environmental conditions. In addition, the rate of allopatric speciation is faster in small populations than in large ones because of greater genetic drift.