Isolating mechanisms

Isolating Mechanisms are features of behavior, morphology, or genetics which serve to prevent breeding between species. Reproductive isolation of populations is established. It is particularly important to the biological species concept, as species are defined by reproductive isolation.

Isolating mechanisms can be divided into two groups, Prezygotic isolating mechanisms and Postzygotic isolating mechanisms

Prezygotic mechanisms

Factors which prevent individuals from mating.

• Geographic isolation: Species occur in different areas, and are often separated by barriers.
• Temporal isolation: Individuals do not mate because they are reproductively active at different times. This may be different times of the day or different seasons. The species mating periods may not match up. Individuals do not encounter one another during either their mating periods, or at all.
• Ecological isolation: Individuals only mate in their preferred habitat. They do not encounter individuals of other species with different ecological preferences.
• Behavioral isolation: Individuals of different species may meet, but one does not recognize any sexual cues that may be given. An individual chooses a member of its own species in most cases.
• Mechanical isolation: Copulation may be attempted but transfer of sperm does not take place. The individuals may be incompatible due to size or morphology.
• Gametic incompatibility: Sperm transfer takes place, but the egg is not fertilized.

Postzygotic isolating mechanisms

An example of reproductive isolation. A mule is the offspring of a horse and a donkey. They are sterile, except in very rare documented cases

Genomic incompatibility, hybrid inviability or sterility.

• Zygotic mortality: The egg is fertilized, but the zygote does not develop.
• Hybrid inviability: Hybrid embryo forms, but is not viable.
• Hybrid sterility: Hybrid is viable, but the resulting adult is sterile.
• Hybrid breakdown: First generation (F1) hybrids are viable and fertile, but further hybrid generations (F2 and backcrosses) are inviable or sterile.

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King, David (2003-04-12). "The Concept of Species". Zoology 304, Evolution. http://www.science.siu.edu/zoology/King/304/species.htm#15.2.3. Retrieved 2007-07-18. 

Mallet, J. L. B. (1998). "Isolating Mechanisms". http://abacus.gene.ucl.ac.uk/jim/Sp/isolmech.html. Retrieved 2007-07-18.