“…The high number of synonymous mutations thus allows protein-coding regions such as groEL to resolve finer-scale differences in diversity than even variable regions of the 16S region can. Of course, selection for particular groEL genes in our treatments is possible; the groEL gene codes for a stress response chaperonin, and mutations therein may confer tolerance to different stress conditions ( 39 , 40 ). However, the vast majority of genetic variability observed in groEL diversity is likely neutral with respect to our treatments and, because HGT of the gene is rare, linked to other regions in the genome that are under selection.…”