Instead of cutting a population in two, a mountain range or a forest could just become an island of inhospitable habitat in the sea of that species’ range.
That’s how being larger might make the tropics more diverse. Now for warmer. As we have seen, the amount of energy pouring into a place is a good guide to the number of species living there. But it seems that the amount of fuel alone cannot account for the difference in diversity. Instead, extra energy might create species by speeding up life’s tempo. This is where metabolic rate re-enters our story.
By influencing metabolism, temperature affects evolution. Organisms whose body temperature matches that of their environments—meaning plants and all animals except for the warm-blooded birds and mammals—have faster metabolic rates in warmer climates and so grow more quickly. They will reach maturity quicker, which will make each generation shorter, and so natural selection will have more to work with. The malaria-carrying Anopheles mosquito, which can crank through 10 generations in a year, is likely to evolve more quickly than a temperate species that can only manage one. In general, species with fast generation times can adapt quickly. That is why pesticide resistance in insects, and antibiotic resistance in bacteria, is such a problem.
Jim Brown’s team has found that the warmer and smaller a species is, the more quickly its DNA changes. Like cellular metabolic rate, mutation rate over time falls as the −1/4 power of body mass. And besides speeding up the life cycle, hot temperatures cause mutations. Male mammals carry their testicles outside their bodies to keep them cool, in an attempt to reduce the mutation rate in their sperm. The effects of temperature on the pace of life, via metabolism, therefore provide a mechanism by which climate can influence the rate at which new species form. About the same amount of energy is needed to spark a mutation in all cells and species. The effect of temperature on mutation does not translate directly into evolutionary change. How much a species alters through time depends on the selective forces it experiences. But it does show that, other things being equal, the DNA