What is a selective constraint? 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?...

It is selective pressure that determines and limits the number of neutral/beneficial mutations that can take place. The more constrained a locus is under selection, the easier it is to generate deleterious mutations and the harder it is to generate beneficial ones; this can be reflected in low substitution rates at such loci in populations with time.

One widely used method to measure this is to compute the Ka/Ks ratio to infer if a locus is selectively constrained or not.