would expect to see there. But although this reasoning helps explain local differences in species diversity, it is less clear whether history—like all the other explanations encountered so far—can explain the global equator-to-pole gradient.
Glaciers do not inevitably wipe out all species in their path. Rather than being eliminated, the temperate zones and their species might simply have shifted to follow the climate. This should be particularly true in the sea, where life is more mobile. And the climatic changes wrought by ice ages also affect tropical forests. Fossil pollen shows that the extent of tropical forests has varied a great deal. It appears that drier and warmer climates lead to large patches of grassland appearing amid the trees. It’s also been argued that isolating clumps of forest from one another promoted diversity, because new species would be more likely to evolve in forest fragments, just as they do on islands. But you can’t have it both ways—either stability promotes greater diversity or it doesn’t. Current conditions seem a better guide to current diversity than past history—the number of species found in a place is correlated more closely to its present climate than to the length of time since that place was last covered in glaciers. There seems to be some balance that, given a few thousand years, restores itself. In the Cretaceous period the world’s climate was much milder. Yet the diversity gradient was still in place. And mass extinctions have hit the tropics just as hard as the rest of the world. It is hard to argue that any one environment is older than any other.
But if the tropics are not biodiversity’s museum, they might be its cradle. There is evidence that, even if they are just as likely to die out near the equator, species are more likely to be born there. Two reasons have been suggested for this phenomenon: the tropics are larger, and they are warmer.
First, larger. It doesn’t always look it on maps, because the Mercator projection typically used to transform the spherical Earth into a two-dimensional map squashes land at the equator and stretches it at the poles, making Greenland look nearly as big as Africa. But looking at a globe, it is clear that the tropics are by far the largest climatic zone on