Nerys Ghemor wrote:
...Unless we make the presumption that some of the humanoid species are either subspecies, or belong to the same genus.
That works for Vulcans/Romulans and MAYBE Humans/Betazeds. We already know the Vulcans and Romulans have a common origin and one might be able to make a case that the same is true with Humans and Betazeds but when you look at species like Human-Vulcan or Human-Klingon or Andorian-Vulcan or Cardassian-Bajoran, forget it. Notwithstanding the scientific illiteracy of some of the posters here, we know how genetics works and we know that natural hybrids between such disparate species is impossible. Even genetically engineered unions are hard to see.
TNG-The Chase was a nice effort to try to fix the mess they created. Unfortunately, the writers either didn't understand how evolution works or they underestimated the scientific knowledge of the typical Trek nerd.
Even if there was a base commonality between all humanoid species, it would still be impossible for disparate species like Humans and Vulcans to reproduce. The physiological differences are enormous. It's not just blood chemistry - although that would be plenty enough to make Human-Vulcan Hybrids impossible. Vulcans have very-high body temperature, very-high heart rate and very-low blood pressure. All the organs would have to function completely differently. As other posters have already pointed out, Humans have FAR MORE in common with great apes than we would with Vulcans and we (thankfully) can't mate with those.