You'll never see a sub-race start out in a new sector unless you've unlocked them first by seeing/helping them emerge. The sub-races arise through unusual circumstances. For example, you might get a quest from the Cortex to help them stop a rebel group of Telepaths. Instead of helping the Cortex, you could help the rebels. Help them enough and they're able to start a revolution and gain control of their own planet, establishing themselves as the new Telepath sub-race.
There are many different ways to unlock the various sub-races. Keep an eye out for unique names (such as Telepaths, Asexuals, Apes, etc.) in the text of the quests you get.