Species Concepts and Reproductive Isolating Barriers
The divergence of species from parental or ancestral populations is reinforced by reproductive barriers that limit exchange of genes with members of other populations. These isolating barriers, originally described by Dobzhanky (1937) as isolation mechanisms, are defined as biological properties of individual organisms that impede gene exchange with members of other populations. External factors, such as geographic barriers are not considered to be reproductive isolating barriers.
Reproductive isolation barriers can be categorized in different ways: relative to the act of mating or relative to the production of zygotes. Premating/prezygotic isolation barriers impede mating between members of different species. However, if members of different species do mate, postmating/prezygotic isolation barriers prevent the formation of a zygote. If these isolating barriers fail and a zygote is formed after the members of two species mate, postmating/postzygotic isolation barriers reduce the viability and/or fertility of the zygote.
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