Angiosperm speciation cools down in the tropics

Abstract

Recent evidence has questioned whether the Latitudinal Diversity Gradient (LDG), whereby species richness increases towards the Equator, results in higher rates of speciation in the tropics. Allowing for time heterogeneity in speciation rate estimates for over 60,000 angiosperm species, we found that the LDG does not arise from variation in speciation rates because lineages do not speciate faster in the tropics. These results were consistently retrieved using two other methods to test the association between occupancy of tropical habitats and speciation rates. Our speciation rate estimates were robust to the effects of both undescribed species and missing taxa. Overall, our results show that speciation rates follow an opposite pattern to global variation in species richness. Greater ecological opportunity in the temperate zones, stemming from less saturated communities, higher species turnover or greater environmental change, may ultimately explain these results.

Recent accelerated diversification in rosids occurred outside the tropics

“…15,16 ) suggesting that nontropical species exhibit high diversification rates despite the lower species richness of communities outside of the tropics. Our results are consistent with recent work that examined speciation rates across geographic space in angiosperms and showed lower angiosperm speciation in the tropics 14 . Our work significantly extends this effort by explicitly considering multiple definitions of tropicality, as well as the relationship between temperature and diversification over geological time and across multiple subclades.…”

“…This association is present both globally across all rosids and in rosid subclades, and is robust to several analytical methods, sensitivity tests, and several data sets representing temperaturespecies associations. Similar patterns of increased diversification with global cooling have been found in mammals 9 , birds 7 , and some other flowering plant lineages 10,11,14,38 . Both falling global temperature and aridification are compatible with the timing of diversification reconstructed here for rosids and most rosid subclades (~10-15 Myr to the present), a timeframe that also corresponds to the origin of many modern-day temperate plant Net diversification rate curves were generated using BAMM for each of the 17 rosid orders (clades) (a-q); each curve is a median line colored by order; the solid line is the original net diversification rate curve, the dotted line represents the rate of tropical lineages, and the dashed line represents the rate of non-tropical lineages (no curves displayed for any lineage with sample number < 3).…”

“…We found compelling evidence of a negative relationship between temperature and diversification in the rosids. These results are part of an emerging pattern in large-scale diversification studies 10,13,14,66 (but see refs. 15,16 ) suggesting that nontropical species exhibit high diversification rates despite the lower species richness of communities outside of the tropics.…”

“…The nature of this relationship has been highly variable, with some studies recovering a negative correlation (e.g., accelerated evolution during cool periods [9][10][11] ) and others a positive correlation (rapid evolution during warm periods 6,12 ). Likewise, studies of latitudinal gradients in diversification rate have found patterns congruent with both of these relationships (i.e., higher diversification at higher latitudes 13,14 , or higher diversification at lower latitudes 15,16 ) or, in some cases, with neither relationship 10,17 . There remains little consensus on the origins of these contrasting biodiversity patterns, with numerous hypotheses proposed across different taxa using different methodological approaches: e.g., the diversification rate hypothesis, the ecological regulation hypothesis, and the tropical conservatism hypothesis [18][19][20] .…”

Geographic patterns and environmental correlates of taxonomic and phylogenetic beta diversity for large‐scale angiosperm assemblages in China

“…We believe that the latitudinal nestedness gradient is caused by the well‐known pattern of decreasing temperature with latitude (Morueta‐Holme et al 2013). While still being debated (Igea and Tanentzap 2020), speciation rate is expected to be high at low latitudes and species ranges tend to be small, leading to different small‐ranged endemic species present in different areas. This would in turn lead to high turnover of species in regions at low latitudes.…”