OK, I think I understand you now. When you unlock a sub-race, it has to survive to the end of the sector in order for it to emerge in the galaxy. Once it has emerged, it has a chance of showing up as a starting race on a new sector.
If the sub-race gets killed off before you complete the sector, then it will not emerge in the galaxy and you won't ever see it again until you repeat the circumstances that made them show up the first time.
If you change to a new galaxy when you start a new sector, then none of the sub-races that have emerged in a previous galaxy will show up in the new one until you unlock them again.