Two closely related species of Chimpanzees mate, the common chimpanzee ( Pan troglodytes) and the Bonobo ( Pan paniscus) are in captivity and given the opportunity to mate. Everything goes well with the mating attempt, courtship is fine, the act of mating is fine, however there is no offspring. Upon further investigation it is determined that the smaller sperm of the common chimpanzee is unable to fully penetrate the larger egg of the Bonobo. This is an example of: