Species that are habitat specialists make up much of biodiversity, but the evolutionary factors that limit their distributions have rarely been considered. We show that in Drosophila, narrow and wide ranges of desiccation and cold resistance are closely associated with the distributions of specialist and generalist species, respectively. Furthermore, our data show that narrowly distributed tropical species consistently have low means and low genetic variation for these traits as compared with those of widely distributed species after phylogenetic correction. These results are unrelated to levels of neutral variation. Thus, specialist species may simply lack genetic variation in key traits, limiting their ability to adapt to conditions beyond their current range. We predict that such species are likely to be constrained in their evolutionary responses to future climate changes.