THE TROPICS AS A SOURCE OF EVOLUTIONARY NOVELTY THROUGH GEOLOGICAL TIME

SPATIAL and temporal variations in biological diversity can be shaped by a variety of dynamical interactions between origination and extinction1-3. For this reason, the evolutionary basis of the latitudinal diversity gradient-with the tropics extraordinarily rich in species, higher taxa and evolutionary novelties-has been much debated4-8. High origination rates with the tropics operating as a diversity pump8-11, low extinction rates with the tropics operating as a diversity accumulator12-15, or some combination of the two16-18, have all been proposed to explain the wealth of higher taxa and morphological variety in low latitudes. Few historical data have been available, however, to test directly whether the tropics are 'a cradle or a museum'15,19. A new palaeontological analysis of post-Palaeozoic marine orders shows significantly more first appearances in tropical waters, whether defined latitudinally or biogeographically, than expected from sampling alone. This provides direct evidence that tropical regions have been a major source of evolutionary novelty, and not simply a refuge that accumulated diversity owing to low extinction rates.