If a particular sub-group is underrepresented, then the conclusions can acknowledge this. The extreme to is to strictly limit any generalizability. These are my group's results. They apply to this group at this time when measured in this way. Strategic retreat should generalize, but should indicate where there are problems.
Universes define the kinds of real populations in which it is relevant to test a particular idea.
There can be a single or multiple criteria which define a universe. Human populations are not presumed to be interchangeable. This is a prudentialist strategy as opposed to an adventurist strategy. A continuum exists between total generalization of results and total specification of results. An appropriate analysis of strategies depends on the conditions of the research question and population as well as the implementation of the research.