In a small population where notables are represented(forest tribes)a chance event (like a natural calamity) might separate the group with out leaving chance a chance for introduction of new allels, then a genetic combination that has a little adaptive value might be favoured .
 
in case of completely dominant genetic diseases then the natural selection might favour recessive allel for example huntington's chorea.
 

 In many cases, the recessive allele will remain in the population but it will not be expressed due to the dominant genes being more frequent.