Which concept refers to differences between species that prevent the transfer of sperm?

The concept that relates to differences between species that prevent the transfer of sperm is structural isolation. This form of isolation arises from physical differences in the reproductive organs of different species, which can make it impossible for them to successfully mate. For instance, if two species have incompatible reproductive structures or mating mechanisms, they cannot transfer sperm effectively during the mating process, thus preventing fertilization.

In contrast, postzygotic mechanisms occur after fertilization has taken place and involve factors that affect the viability or reproductive capability of hybrid offspring, which does not pertain to sperm transfer. Hybrid breakdown refers specifically to the reduced fitness of subsequent generations of hybrids, rather than to the initial mating process. Behavioural isolation entails differences in mating behaviors or rituals that prevent species from recognizing each other as potential mates, rather than structural factors influencing sperm transfer. Therefore, structural isolation is the most appropriate term for the situation described in the question.