The more isolated or insular the community, the less likely there is for inter-species off spring.
Actually, I'm not quite sure this is true. I have read studies before projecting about the multiracial future of mankind, and among the last places to have people visibly one race or another will be densely populated areas like Central China. The reason being that there are just so many people of a single ancestry that the occasional person moving in is like a drop in the bucket, with ancestry more or less vanishing within a few generations.
In addition, it's pretty easy for areas like remote mountain valleys to experience genetic drift over time. Just have one successful outsider move into a small town, have lots of children, and in a few hundred years half of the population could end up with his last name.
Trek has generally speaking always been terrible depicting a reasonable amount of multiracial people - whether human or human-alien hybrids. I suppose one could partially chalk this up to presentism, and partially to the vagrancies of casting. But if I were casting director of a Trek show, I would make 80% of the humans an indeterminate brown.