Reproductive Isolation
Reproductive isolation is also called hybridization barriers. It refers to a collection of different mechanisms, psychological processes and behaviors that prevent members or organisms of two distinct species that mate or cross from generating or producing an offspring or ensures that the offspring that can be produced by the individuals is not sterile.
Reproductive isolation maintains integrity of species with time by directly impeding or reducing flow of genes between the individuals of distinct species. This allows the characteristics of each species to be conserved.
Reproductive isolation mechanisms have been categorized in different ways. For instance, Ernst Mayr, a zoologist classified reproductive isolation mechanisms in two different categories. These are the ones that act prior fertilization or prior mating in animals. These are known as pre-copulatory.
The other category comprises of those acting after fertilization or mating in animals. The mechanisms have been classified as pre-zygotic and the post-zygotic. Various reproductive isolation mechanisms are controlled genetically and this has been demonstrated using experiments that there can be an evolution in a species with overlapping geographic distribution or sympatric speciation or due to the adaptive divergence that usually accompany allopatric speciation.
In regards to population’s biological efficiency, pre-zygote isolation mechanisms of reproductive isolation are very economic. This is because resources are not lost or wasted on production of weak, sterile, or non-viable descendent.
There is also habitat or temporal isolation. This refers to any factor that prevents individuals that are potentially fertile from meeting reproductively isolating the members of that distinct species. Among the type of barriers that may cause this form of isolation include physical barriers, difference in time of flowering or sexual maturity and different habitats. Species often branch off when the factors change. This is commonly seen when physical barrier is altered.
Another form of reproductive isolation is sexual isolation through conduct or behavior. This includes different rituals during mating by animal species which can create extremely powerful barriers to reproduction. These are called behavior or sexual isolation and they isolate similar species in most groups in a specific animal kingdom.
Mechanical isolation can also contribute to reproductive isolation. This occurs when mating pairs are unable to successfully couple because they have incompatible genitals. Evolution which has resulted in the development of organs that are increasingly complex with divergent characteristics causes mechanical isolation among species. Other types of reproductive isolation mechanisms include gametic isolation, hybrid sterility, and fertilization isolation mechanisms among others.
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Sources
http://evolution.berkeley.edu/evolibrary/article/evo_44
http://www.bio.miami.edu/dana/dox/reproductive_isolation.html
http://en.wikipedia.org/wiki/Reproductive_isolation