Dodd, 1989
Reproductive Isolation- There are barriers that cause one species to not have gene flow, so genes are not able to be mixed in that population. This leads to speciation.
These "barriers" can be external such as the environment or internal barriers.
Allopatry
There are two mechanisms of reproductive isolation: pre-copulatory and post-copulatory (Mayr, 1963). They can evolve in species whose geographic distribution overlaps (sympatric speciation) or as the result of adaptive divergence that accompanies allopatric speciation (geographic separation). Allopatric separation occurs when two population become isolated from each other (vicariant).
Mating location, time and rituals
Genetically-based changes to these aspects of mating could complete the process of reproductive isolation and speciation. For example, bowerbirds (shown below) construct elaborate bowers and decorate them with different colors in order to woo females. If two incipient species evolved differences in this mating ritual, it might permanently isolate them and complete the process of speciation (http://evolution.berkeley.edu/evolibrary/article/evo_44 ).
References
Mayr, E. 1963. Animal species and evolution. Harvard University Press, Cambridge.