The Gondwanan legacy in South American biogeography

The tropical conservatism hypothesis (TCH) suggests that phylogenetic niche conservatism (the tendency for traits to be maintained during diversification) should be the main driver of latitudinal diversity gradients. For example, the tropical-temperate diversity gradient for woody angiosperms should reflect the tropical, older origin of most clades, the constrained evolution of cold tolerance, and the fact that few clades were able to persist in temperate zones after the beginning of the global cooling in the Eocene. Evidence for this explanation has been discussed primarily from a Northern Hemisphere perspective. Recently, two studies have presented diversity and clade-age relationships in South American forests that are inconsistent with the TCH, with older woody families being more frequent at higher latitudes and higher elevations in the Andes. We argue that a broader framework considering the ancient history of the Southern Hemisphere flora and the recent history of the Andean flora can help explain these discordant patterns. Here, we provide an initial discussion of this new perspective, emphasizing the historical development of a unique and rich palaeoflora of Gondwanan ancestry at mid-to high latitudes of South America and Antarctica. We suggest that the idea of Austral niche conservatism (ANC) of a warm-temperate Antarctic-South American biota should be explored further to improve our understanding of biogeographical patterns in the Southern Hemisphere.