And it only gets worse when you bring in organisms that reproduce asexually, like bacteria. Then there’s no interbreeding criterion at all, and many bacteria (or even wildflowers to a lesser extent) readily trade genes (for plants, even whole chromosomes) horizontally among species.
As for orcs and fictional halfbreed races like half-elves and half-goblins, the best explanation I’ve see actually came from HPMOR:
“I mean -” Harry said even more quietly, trying to figure out how to ask whether goblins had evolved from humans, or evolved from a common ancestor of humans like Homo erectus, or if goblins had been made out of humans somehow - if, say, they were still genetically human under a heritable enchantment whose magical effect was diluted if only one parent was a ‘goblin’, which would explain how interbreeding was possible, and in which case goblins would not be an incredibly valuable second data point for how intelligence had evolved in other species besides Homo sapiens - now that Harry thought about it, the goblins in Gringotts hadn’t seemed very much like genuinely alien, nonhuman intelligences, nothing like Dirdir or Puppeteers - “I mean, where did goblins come from, anyway?”
“Lithuania,”
