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The mechanisms of reproductive isolation have been classified in a number of ways. Zoologist Ernst Mayr classified the mechanisms of reproductive isolation in two broad categories: pre-zygotic for those that act before fertilization (or before mating in the case of animals) and post-zygotic for those that act after it. [5]
Examples of pre-zygotic isolation include habitat isolation, isolation via pollinator-pollination systems, and temporal isolation. Examples of post-zygotic isolation involve genetic incompatibilities of hybrids , low fitness hybrids, and sexual selection against hybrids.
The species contact zones exhibit, "strong postzygotic selection against hybrids" and enhanced isolation from differences in mating call. [ 13 ] An alternative to detecting reproductive character displacement in populations that overlap in sympatry is measuring rates of hybridization in contact zones. [ 9 ]
Reinforcement is a process of speciation where natural selection increases the reproductive isolation (further divided to pre-zygotic isolation and post-zygotic isolation) between two populations of species. This occurs as a result of selection acting against the production of hybrid individuals of low fitness.
Pre-zygotic reproductive isolation means that the reproducing individuals in the populations were unable to produce offspring (effectively a positive result). Post-zygotic isolation means that the reproducing individuals were able to produce offspring but they were either sterile or inviable (a positive result as well).
The Dobzhansky–Muller incompatibilities can result from purely random, neutral or non-selected differences between the populations. They can also be driven by natural selection in at least two ways. When two populations diverge from each other and encounter new - and different - environments they may adapt to these environments.
Support for the reproductive character displacement hypothesis comes from observations of sympatric species in overlapping habitats in nature. Increased prezygotic isolation, which is associated with reproductive character displacement, has been observed in cicadas of genus Magicicada, stickleback fish, and the flowering plants of the genus Phlox.
Under this definition, it includes pre-zygotic divergence and complete post-zygotic isolation. [63] Maria R. Servedio and Mohamed Noor consider any detected increase in pre-zygotic isolation as reinforcement, as long as it is a response to selection against mating between two different species. [64]