Figure 1.

Latitudinal and elevational patterns of temperature variation and overlap as predicted by Janzen's (1967) hypothesis. The annual range of ambient temperatures experienced by low and high elevation sites is depicted for (a) temperate and (b) tropical montane regions. Greater temperature seasonality in temperate regions results in high overlap in the temperatures experienced by organisms at low and high elevation sites. In contrast, sites separated by the same elevations in the tropics show much less overlap owing to the lack of temperature seasonality. Janzen hypothesized that these climatic differences lead to the evolution of organisms with broadly overlapping thermal and physiological tolerances at low and high elevations in temperate mountains, resulting in greater opportunity for dispersal between elevationally separated sites in the temperate zone compared with the tropics. Conversely, in the tropics, it is more difficult for organisms to disperse between different elevations, leading to greater opportunities for climatically driven speciation along tropical mountain slopes.