The old way of doing this, in a hurry, is to in-breed, keeping the females pregnant constantly.
The defective were killed. Those who could be useful could be kept alive, but if male, not allowed to breed. That way, some out breeding of the undesirable traits might be accomplished.
Once the population had reached a size that enabled survival, then incest would be banned. The range of genetic variety would have been concentrated into what would become a "race".
This would be necessary after a catastrophe that threatened the survival of that species. The Y chromosome would exhibit a very tight range, suggesting only a few males.
Encounters with others would be encouraged and gifts and trade on equal terms would also flourish. Adventures and anger would be curtailed as much as possible. Flight rather than fight for at least four generations. Sea travel would be favoured for the contact and evasion potential.