These "barriers" can be external such as the environment or internal barriers.
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 ritualsEdit
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 ).
Mayr, E. 1963. Animal species and evolution. Harvard University Press, Cambridge.