We identify as a group - a tribe. In recent millenia, it's worked out well for individuals to identify with even larger groups. Today, it's rational to identify globally, for the benefits of trade, collaborative technological development and peace. There may well be some evolutionary pressure towards this. (Yes, there's reward for defection, but collaboration works even better).

Granted, "global" isn't the same as "species" - but in practice, and forever there is communication between members, they are identical (have the same referent).