be tested. Our understanding of how temperature, via metabolic rate, influences development and mutation provides a mechanism for more rapid tropical evolution. And in some senses it looks good by default. Most other reasons for the diversity gradient have been around for a while, giving people time to point out limits to their generality. Ideas about evolutionary rates are relatively new and have yet to be ground down by the mill of academic scrutiny.
After two centuries of research, a toolkit of ideas (or a bunch of hedged bets) is starting to emerge that might enable us to explain why there are more species in one place than another and why there are more species in the tropics than anywhere else. Climate supplies energy, which influences evolution. Climate also influences the harshness and productivity of different environments, which controls what species can live there. History, in the time needed for diversity to recover from past shocks, is surely a factor, as is the world’s geometry, because the tropics are larger than anywhere else and because they are in the middle of their domain.
How climate, history, and geometry might interact to control biodiversity is still poorly understood and controversial. Their combined effect is bound to be complex and will vary from place to place. It may be that the simple, top-down approach favored by macroecologists can only get us so far. One possible way forward is through computer simulations. Satellites provide the raw material for such models, in the shape of data on temperature, rainfall, solar radiation, and vegetation cover, which can then be matched to patterns in diversity, particularly the diversity of well-studied groups such as birds and mammals. These models will not be simple, like the 3/4 power law. They will be more like the models that climate scientists use to predict the movements of air and water around the atmosphere. These simulations require vast computer power and give ambiguous results, with sizable errors. Some ecologists are already employing such models to see how their theoretical predictions match up with reality and to forecast how species might respond to climate change. Perhaps the Alexander von Humboldt of the twenty-first century will set sail on a computer.