There is a wealth of genome material available for the model organism, Drosophila melanogaster. Work in the area has elucidated a range of immune genes under positive selection. However, a broader question is whether there are interactions between Drosophila genetics and the environment. Using PAML, we found that purifying selection works most strongly on Drosophila spp. found primarily in a narrow geographical distribution, namely in tropical species. This work suggests that there may be more interesting interactions to be plumbed from other data, particularly in vector datasets. If similar trends are seen in virus vectors, this has implications for the spread of disease and vector ability to respond to pathogens as the climate warms.