I encountered the term selective constraint in Huber et al. 2015, page 4 (last paragraph) in:
If invariable sites are included in the analysis, then both the methods of Kim and Stephan (2002) and Nielsen et al. (2005) may be sensitive to assumptions regarding selective constraint and mutation rates.
but the context does not quite help.
What it the definition of selective constraint?
Does it has to do with lethal mutations in non-coding regions? Does it has to do with selection interference?...