of small, hot, fast-burning plants and animals will change more quickly than that of large and cool living things. Metabolism, via its affect on the rate of life, provides a mechanism by which climate can influence the rate at which new species form.

Brown’s team used temperature and metabolic rate to build a model of diversity variation with temperature, based on temperature’s affect on metabolism (note that this says nothing directly about the rate of mutation or the appearance of new species). Patterns in the diversity of trees, amphibians, freshwater fish, marine molluscs, and the parasites of marine fish all seem to match the model’s predictions, with a place’s diversity rising as it gets warmer.

Fossil evidence supports the idea that the tropics are evolution’s workshop. David Jablonski has shown that most groups of marine animals first appear in equatorial rocks. Many temperate species—including us—are tropical migrants, not creatures born and bred in the cold. Tropical rocks also contain more young species than temperate rocks, suggesting that evolution works faster close to the equator. And from looking at fossil foraminifera, Drew Allen, a former student of Brown, has found that tropical species, which have metabolic rates 15 to 20 times those of their polar cousins, evolve new species more rapidly than their cold-water counterparts. Tropical forams also go extinct more quickly, perhaps because if one species splits into two, each new species has a smaller population. By working out at the molecular level how much energy is needed to cause a mutation and the amount of time it takes for a new species to evolve, Allen has made a back-of-the-envelope calculation that it takes 10²² joules to evolve a new species of foraminifera. About the same amount of energy shines down as sunlight on Earth each day.

The link between temperature, metabolism, and evolution is a good candidate for an explanation of why there are more species in the tropics. It is one of the few relationships between environment and biology that ought to hold for every species, wherever it lives; many of the other explanations were derived from observing land plants and animals, and they founder when applied to marine life. It seems to fit with fossil evidence. It predicts roughly how many species we should expect to find in a place of a given temperature—rather than just offering a reason for the shape of the trend—which allows its ideas to