There are many factors that drive speciation, most of which involve the isolation of one population from another (Tregenza & Butlin, 1999). These drivers of isolation can be behavioral, geographical, physiological, temporal, biological and mechanical. One form of isolation is gametic isolation.
Gametic isolation is a fairly broad term that is inclusive of all reproductive barriers between mating and fertilization of the egg (Ludlow & Magurran, 2006).When we think about hybrids between species who are able to successfully breed and have offspring, the example of the mule comes to mind, the result of a cross between a female horse and a male donkey. While in this case, it is biologically possible for two different species to breed successfully, the offspring of that pairing is unable to breed. This type of sterility is an example of gametic isolation. Gametic isolation can also be seen in many aquatic species which release their gametes into the water and fertilization takes place externally. Hybridization is extremely rare in these cases, even though there is no physical barrier preventing cross-species fertilization (Campbell and Reece).
Gametic isolation is an important barrier in between plant species which reproduce by pollination, again in the absence of other barriers to prevent cross-pollination. Gametic isolation is one way that genetic integrity is preserved between two species who are either able to breed successfully or where there are not other barriers in place to limit cross fertilization.
Campbell and Reece. Biology. Chpt. 24: Species and Speciation.
Ludlow, A. M., & Magurran, A. E. (2006). Gametic isolation in guppies (Poecilia reticulata). Proceedings of the Royal Society B: Biological Sciences, 273(1600), 2477-2482.
Tregenza, T., & Butlin, R. K. (1999). Speciation without isolation. Nature, 400(6742), 311-312.