ANALYZING THE LATITUDINAL DIVERSITY GRADIENT: TESTING THE "AGE OF THE TROPICS" HYPOTHESIS USING DEEP TIME

The latitudinal diversity gradient, in which species' richness decreases away from the tropics, is one of the most well-studied patterns of global diversity. The pattern is evident among many organisms, and in both marine and terrestrial realms. Although several hypotheses have been suggested to explain the gradient, no consensus exists as to what process leads to the pattern.

Recently, the "Age of the Tropics" hypothesis (AOTH) has come to the fore. This hypothesis is based on the ideas that (a) tropical climates are older, and that colder climates are a relatively recent phenomenon; and (b) young, ecologically dominant clades evolve in the tropics, and subsequently expand their geographic ranges. For example, heteroconch bivalves, which are one of the youngest and most successful bivalve clades, have a steeper gradient than other bivalves (Crame, 2000). The AOTH suggests that the diversity gradient exists because dominant clades have not had sufficient time to adapt to cool climates and to expand their ranges.

To test if the "Age of the Tropics" hypothesis can be generalized, I examined the distributions of productidine brachiopods through the Pennsylvanian. Although the diversity gradient has existed at many points in time (Stehli et al., 1969), only rarely has it been studied through a time interval. The Pennsylvanian and Recent are similar in climate, sea-level fluctuations, and tectonism. Productidina is one of the most ecologically successful, and youngest, of brachiopod clades. If the AOTH is correct, then productidines should manifest a pattern of increasing geographic range over time. A biogeographic database of Carboniferous productidines (n=317, temporal resolution=stage) reveals no trend of continued expansion. During the Pennsylvanian, there was no significant change in latitudinal range within individual genera (McNemar's Chi-square). Furthermore, a phylogeny of the productidine clade Overtoniidae indicates that no branch entails a Pennsylvanian expansion of a tropical lineage into temperate regions. In contrast, two previously widespread lineages contract their latitudinal ranges during the general cooling trend after the Visean. This result suggests that climate, not time, may drive latitudinal expansion, and casts doubt on the "Age of the Tropics" hypothesis.