I know that for two populations A and B, there are situations in which there is a two way exchange of individuals between the populations, and there are situations in which there is no exchange of individuals between the populations. I have thought before of a situation involving one way isolation between two populations, in which it's easy for individuals from population A to immigrate to population B, but impossible for members of population B to immigrate to population A.
I tried searching to see if I could find any examples of one way isolation between populations, and couldn't find any examples, although I don't know if this is from not using the correct keywords to find it, not knowing where to look, or there being no known examples of it.
Are there any real world examples of the one way isolation I mentioned between populations?