Horizontal gene transfer plays a vital role in bacterial evolution, primarily by spreading adaptive genes between cells both within and between species. Genes capable of HGT are generally encoded on mobile genetic elements such as plasmids, and therefore have a potential advantage over those encoded exclusively on the chromosome, as mobile genes can be spread both vertically and horizontally. This thesis investigates how gene mobility affects the spread of resistance genes in bacterial populations and communities, and how this process is affected by the environmental and community context. First, I demonstrate that being encoded on a mobile plasmid widened the range of parameter space in which resistance genes could spread, allowing resistance to reach fixation in the absence of positive selection, through horizontal transmission of the plasmid. Secondly, I show that high frequencies of pulsed positive selection increased plasmid stability in the population, but that under low frequencies of pulsed positive selection, horizontal transmission played a greater role in plasmid stability. Thirdly, I showed that the presence of lytic bacteriophages limited the selective conditions under which mobile plasmid-encoded resistance could persist in a population, but that sufficiently high rates of horizontal transmission could overcome this cost. Finally, I showed that mobile plasmid-encoded resistance genes were able to horizontally transfer into a natural soil community, enabling the survival of a more diverse portion of the community in response to lethal environmental change. Overall, this thesis demonstrates how environmental context favours the spread of mobile versus non-mobile resistance genes, and provides insight into the consequences this may have for the evolutionary potential of the wider bacterial community.