Significance Bacteria can obtain genes from other bacteria, or the surrounding environment, by horizontal gene transfer (HGT). While it is clear that HGT is very important for microbial populations, it is not understood how HGT changes the rate or mechanisms of adaptation. In this study, we evolve populations of the bacteria Helicobacter pylori and use DNA sequencing to track the movement of HGT genes as they spread through the population. We show that HGT can help antibiotic resistance genes establish at a low frequency in a population, even in the absence of the antibiotic. We find that these HGT treatment populations flourish when treated with antibiotics, showing how HGT can potentiate adaptation to future environmental change.