system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: how have small tribes ensured or maintained genetic diversity?
gpt
A lot of them haven't, at least not to what we'd consider to be an optimal level.  And that's not always guaranteed to be a problem.  

While a population can eventually reach a point where inbreeding has created so many birth defects that children rarely live to adulthood, it takes a very long time for that to happen unless there was already a strong predisposition for those defects present within the gene pool.  In the short term, a loss of genetic diversity will just make a population more vulnerable to being wiped out by external factors like a new pathogen, sudden climate shifts, or the depletion of some vital resource.  Humans tend to be uniquely talented at anticipating and avoiding these risks in our day to day lives compared to most species living on the planet, meaning it's a lot easier for us to endure even with that heightened vulnerability.