Disadvantages [of the biological species concept, which defines a species as a reproductively isolated community]:
• Exceptions exist, different species sometimes do interbreed … Captain Kirk?
Like on one hand it’s charming that the professor is trying to connect…
Isn’t Spock half-human? That’s an example of postzygotic interaction there, if I’m understanding it correctly.
You pretty much are. The thing I’m caught up on is the bit where he had to be actually genetically engineered deliberately, if I remember correctly. Postzygotic isolation means that even if you get mating between two species, they’re not as fertile with each other as they are with themselves—eggs may be less likely to fertilize, or the offspring may be less likely to be viable or the adult offspring may not be fertile. So to say there’s no postzygotic interaction between humans and Vulcans, Spock would have had to be naturally conceived and carried to term without extreme medical intervention. And Spock *himself* would have to be capable of reproduction with at least one of his parent species.
Postzygotic isolation can also be sex-specific, as in Haldane’s Rule—the heterogametic sex (male mammals, female birds—XY or ZW individuals) is more likely to suffer from hybridization effects than the homogametic sex. So for example, there are known examples of fertile female ligers, but the males are always sterile, and any introgression of tiger genes into lions (or vice versa) would have to come from female offspring backcrossing into one of the parent species.