Alein - Well said, and sound.
I'm certain you're right - since reproduction of viable offspring has historically been supremely difficult, and since reproduction is by definition an intra-species capability, it really could not be other than that organisms act to maintain proximity with those like them and distance from those unlike them. Species that don't do that are the ones that aren't around any more. The evolutionary premium on fruitfulness and multiplication being as terribly high as it has been, it is an evolutionary waste of time ( and a losing adaptive strategy) for lions, as an example, to invest even a second on chatting up tigers. Given the severe penalities associated with failure to multiply, one would suppose that succesful species err toward excessive discrimination: rejecting viable mates for differences which would not effect reproduction. Perhaps this is the origin of what some call racism nowadays. I'm not sure that's the right word : aren't '-isms' to do with cognition, belief, etc. ?
I have no dog in the aquatic ape debate. I've said I like the theory for esthetic and hedonistic reasons, but as a scientist I take it and all other theories on a provisional basis pending more data !