The Relative Roles of Selection and Drift in Phenotypic Variation: Some Like It Hot, Some Like It Wet

Acoustic signals mediate important functions, e.g. orientation, foraging and communication, that impact on the survival and reproduction of animals. The propagation of acoustic signals is also known to be influenced by habitat, particularly differences in climate. It is therefore likely that the environment would exert significant influence on such signals and that selection rather than drift would be largely responsible for geographic variation in acoustic signals. We investigated the role of selection and drift in geographic variation in the echolocation of two species of horseshoe bats Rhinolophus damarensis and R. clivosus (Rhinolophidae) with wide geographic distributions across the arid and mesic biomes of southern Africa. In both species, selection was found to be the dominant evolutionary process influencing phenotypic variation; however, there was evidence of drift in R. clivosus. Furthermore, selection was not differentially exerted across populations because there was no change in the results when localities were excluded one at a time. Population divergence appeared to be mediated by selection on traits associated with manoeuvrability, detection and size in both species despite their disparate distributions. However, the climatic factor that best explained geographic variation in echolocation was dependent on the biomes occupied by the species. Temperature was the dominant climatic factor in R. damarensis, a species with a largely arid distribution. In R. clivosus, a species with distributions across both mesic and arid biomes, temperature and relative humidity together explained variation in echolocation.