388 NATURE GEOSCIENCE | VOL 3 | JUNE 2010 | www.nature.com/naturegeoscience news & views S ince the dawn of animal life, the rates at which new genera originate have varied spatially, temporally and even by environmental setting 1–3 . Much disputed, however, are the reasons why such patterns emerge. Biological factors, such as competition between groups and the need to evolve new defences, are oſten the first processes invoked. Yet the influence of abiotic environmental factors remains unclear. Writing in Nature Geoscience, Cárdenas and Harries 4 suggest that global changes in nutrient concentrations are an important trigger of changes in marine origination rates. Previous models of origination have tended to highlight biotic interactions 2 and stochastic events 5 . e importance of biotic interactions is exemplified by the observation that episodes of mass extinction are usually followed by phases of mass origination 2 . is suggests that factors such as competition for resources or predation pressure must prevent successful origination in the absence of elevated extinction rates. Cárdenas and Harries 4 assessed whether abiotic factors have an additional role. ey looked for correlations between the rate at which new genera of marine animals emerged in the fossil record and various proxies of environmental conditions in ancient seas. e strongest positive correlation appeared between origination and proxies that are indicative of continental weathering and nutrient recycling. ey also found a strong negative correlation between origination rates and sea level. Both the exposure of continental shelves on falling sea levels and higher rates of continental weathering would increase the delivery of nutrients to the oceans. Based on these correlations, Cárdenas and Harries suggest that high nutrient levels may have promoted origination in the oceans. is conclusion is, at first glance, intuitive, because long-term increases in marine nutrient concentrations seem to go hand-in-hand with rising productivity, biomass and biodiversity 6,7 . However, origination is oſten independent of diversity 2 . Rises in nutrient concentrations would actually be expected to affect previously identified biological factors of speciation in a way that contradicts Cárdenas and Harries’s findings. Specifically, higher productivity and biomass should, by means of larger population sizes, actually inhibit genetic diversification 8 . is is in accordance with data from the fossil record, which suggest that small populations not only have a higher risk of extinction but also a greater potential for origination 9 . Moreover, several studies have documented that, compared with nutrient-rich coastal settings, marine origination rates were significantly higher in carbonate systems with lower nutrient availability 3,10,11 . It is conceivable that, on geological timescales, high nutrient concentrations supported a greater average metabolic activity rather than just larger population sizes. is could allow a more complex realization of ecological niches and a more active occupation of new ecological spaces 12 , which could enhance origination rates. But greater metabolic rates can also be achieved at the same nutrient levels if average population sizes become EVOLUTION Promoting marine origination The rate at which new marine animals evolve has varied through time, but the factors that ultimately drive these fluctuations are unclear. A statistical analysis shows that global changes in abiotic factors play an important role. Wolfgang Kiessling O S D C P Tr K J Pg N 400 300 200 100 0 Age (Ma) 2.0 1.5 1.0 0.5 0.0 Origination rate 0.7090 0.7085 0.7080 0.7075 0.7070 87 Sr/ 86 Sr Caledonian orogeny Variscan orogeny Pangaea Alpine–Himalayan orogeny Figure 1 | Variations in origination rates and the seawater strontium isotope composition. The relatively strong relationship between the origination rates of marine genera (green) and the inferred runoff of continental nutrients (red) led Cárdenas and Harries 4 to propose that enhanced nutrient input is an important trigger of marine evolution. The peaks in continental runoff also coincide with significant tectonic events, which increased sediment delivery and formed geographic barriers that could have triggered speciation. The dashed lines indicate the timing of the five largest mass extinctions, which preceded pulses of origination. © 20 Macmillan Publishers Limited. All rights reserved 10