The distribution of fitness effects of HGT variants. Regression of fitness values for a set of indel mutations identified in double-drug–selected population E3 and measured in growth media with clarithromycin (y axis) and growth media with clarithromycin and metronidazole (x axis) (A) (Materials and Methods and Dataset S2). The distribution of HGT block length for four different evolved populations (E3, F1, G5, and F4) measured growth media without antibiotics (B). The distribution of fitness effects of blocks of HGT variants, for HGT treatment populations measured in the same conditions as their evolution treatment; growth media without antibiotic (C and D), growth media with clarithromycin (E), or clarithromycin and metronidazole (F).