Latitudinal diversity gradients: Phylogenetic signal among flowering plants

Tropical areas have long been known to contain higher species diversity than comparable temperate areas, but why this pattern exists remains a matter of debate. Using a data set from flowering plants, a highly diverse group of organisms, we test whether environments of origin can affect patterns of diversity. Two patterns are possible: (1) clades diversify at higher rates in the tropics, even when compared only to temperate groups of the same age. (2) Tropical diversity reflects tropical origins of flowering plants, with paraphyletic groups making up the bulk of tropical diversity. Under this scenario, tropical and temperate clades of equal age will have comparable species richness. We use phylogenetically based sister-pair comparisons to test this hypothesis, and show that when equal-age temperate and tropical taxa are compared, temperate clades are not less speciose than their tropical sisters. For groups that included both tropical and temperate members, maximum-likelihood reconstruction of ancestral habitat was used to estimate at what point in the evolutionary tree shifts took place. In most cases, these reconstructions confirmed that temperate clades are nested within larger paraphyletic tropical groups. Our results highlight the importance of historical analyses in fully understanding contemporary patterns of species diversity.