Upper thermal limits of Drosophila are linked to species distributions and strongly constrained phylogenetically

Abstract

Upper thermal limits vary less than lower limits among related species of terrestrial ectotherms. This pattern may reflect weak or uniform selection on upper limits, or alternatively tight evolutionary constraints. We investigated this issue in 94 Drosophila species from diverse climates and reared in a common environment to control for plastic effects that may confound species comparisons. We found substantial variation in upper thermal limits among species, negatively correlated with annual precipitation at the central point of their distribution and also with the interaction between precipitation and maximum temperature, showing that heat resistance is an important determinant of Drosophila species distributions. Species from hot and relatively dry regions had higher resistance, whereas resistance was uncorrelated with temperature in wetter regions. Using a suite of analyses we showed that phylogenetic signal in heat resistance reflects phylogenetic inertia rather than common selection pressures. Current species distributions are therefore more likely to reflect environmental sorting of lineages rather than local adaptation. Similar to previous studies, thermal safety margins were small at low latitudes, with safety margins smallest for species occupying both humid and dry tropical environments. Thus, species from a range of environments are likely to be at risk owing to climate change. Together these findings suggest that this group of insects is unlikely to buffer global change effects through marked evolutionary changes, highlighting the importance of facilitating range shifts for maintaining biodiversity.

Fig. 1.

Phylogenetic hypothesis for 94 Drosophila species. CT_max was mapped onto the phylogeny using ancestral trait reconstruction via maximum likelihood; the 95% confidence intervals are shown in parenthesis. Branches were color-coded according to their likelihood states; color groups were determined by dividing all 94 species into 12 equal-sized groups.

Fig. 2.

Species and distribution safety margins. The “Central” species safety margins were calculated as the difference between heat tolerance (CT_max) and the average of maximal environmental temperature (T_max) (filled black circles). To include the species-specific populations that experience more severe heat stress we also calculated the “distribution” safety margins using the assumption that these populations experience a T_max equivalent to average + SD of T_max. Species present on both hemispheres are represented for the northern and southern hemisphere. The species are separated [temperate (black edge), humid tropical (green edge), and dry tropical (blue edge)] according to their average environmental habitat (see text).