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Link to original content: http://pubmed.ncbi.nlm.nih.gov/37159475/
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. 2023 May 16;120(20):e2220672120.
doi: 10.1073/pnas.2220672120. Epub 2023 May 9.

The build-up of the present-day tropical diversity of tetrapods

Ignacio Quintero  1 , Michael J Landis  2 , Walter Jetz  3   4 , Hélène Morlon  1
Affiliations

The build-up of the present-day tropical diversity of tetrapods

Ignacio Quintero et al. Proc Natl Acad Sci U S A. .

Abstract

The extraordinary number of species in the tropics when compared to the extra-tropics is probably the most prominent and consistent pattern in biogeography, suggesting that overarching processes regulate this diversity gradient. A major challenge to characterizing which processes are at play relies on quantifying how the frequency and determinants of tropical and extra-tropical speciation, extinction, and dispersal events shaped evolutionary radiations. We address this question by developing and applying spatiotemporal phylogenetic and paleontological models of diversification for tetrapod species incorporating paleoenvironmental variation. Our phylogenetic model results show that area, energy, or species richness did not uniformly affect speciation rates across tetrapods and dispute expectations of a latitudinal gradient in speciation rates. Instead, both neontological and fossil evidence coincide in underscoring the role of extra-tropical extinctions and the outflow of tropical species in shaping biodiversity. These diversification dynamics accurately predict present-day levels of species richness across latitudes and uncover temporal idiosyncrasies but spatial generality across the major tetrapod radiations.

Keywords: Bayesian inference; biodiversity; biogeography; diversification; tropics.

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Conflict of interest statement

The authors declare no competing interest.

Figures

Fig. 1.
Fig. 1.
Latitudinal tetrapod richness and diversification patterns. (A) Map of tropical (T) and extra-tropical (E) delimitation. (B and C) Tetrapod total richness and their present-day speciation rates (λtip, an average over 10 trees) aggregated by 10° wide latitudinal bands (D–F) Tetrapod total richness, richness density (per km2), and present-day speciation rates aggregated by tropical and extratropical regional delimitation (endemics are shown as solid bars). In (C and F), boxplots show the range (dotted whiskers), 95% quantile (solid whiskers), 50% quantile (box), and absolute median (black line). Circles represent the median weighted by species range size. (G) Posterior distributions (with density standardized across all parameters for visualization) across 10 trees for each tetrapod clade for speciation in the tropics (λT), extra-tropics (λE), and widespread lineages (λW), extinction in the two regions (μT & μE), and dispersal (δ) from the tropics to the extra tropics (T → E) and vice versa (E → T) for model “G”, showing no difference in speciation rates between tropical and extra-tropical regions, higher extinction rates in the extra-tropics, and higher dispersal out of than toward the tropics. (H) Bayes factors (BF) between “G” models with equal or distinct rates in speciation, extinction, and dispersal (an arrow shows the direction if one or more BFs exceed > 1010 in magnitude). (I) Lineage through time (LTT) plot for each region and each tetrapod clade as the sum over lineages of the posterior marginal ancestor state probabilities through time (SI Appendix).
Fig. 2.
Fig. 2.
Heterogeneity in diversification rates within regions. Results of the “G+H” model, which includes two hidden states (0 and 1) that represent two rate categories within each of the geographical regions that are not related to any observed character. (A and B) Posterior distributions (with density standardized across all parameters for visualization) across 10 trees for each tetrapod clade for (A) net diversification rates (λ − μ) in the tropics (Top) and extra-tropics (Bottom) and for (B) net rate flow out of the Tropics (δ(T → E) − δ(E → T)), for the two hidden states (0: up-facing and 1: down-facing). Lineages in hidden state 1 have similar net diversification rates in the tropics and extra-tropics, and higher net diversification rates than those in hidden state 0 in both the tropics and the extra-tropics. In the extra-tropics, lineages in state 0 have a negative net diversification rate. (C and D) Tip-speciation rate (λtip) versus corresponding marginal posterior tip probabilities of being in hidden state 1 for (C) tropical and (D) extra-tropical species. Lineages with a high present-day speciation rate tend to be in the state with higher deep-time net-diversification rates (i.e., state 1). Clade colors as in Fig. 1.
Fig. 3.
Fig. 3.
Results of the “G+E+H” model, which includes two hidden states (0 and 1) and allows for paleoenvironmental data to shape speciation rates. (A) Maps every 40 My of tropical and extra-tropical terrestrial area. (B) Tropical and extra-tropical area reconstruction, (C) their respective change (logarithmic derivative with respect to time), (D) temperature, and (E) its rate of change for the last 200 My. (F and G) “G+E+H” rate estimates in the tropics and the extra-tropics for each of the two hidden states (0 and 1) according to the best-selected paleoenvironmental models with time-varying speciation (F) and time-constant extinction (G) (amphibians: area “A(t)” and exponential time dependency “ETD”, reptiles: temperature “T(t)” and “A(t),” birds: “A’(t),” mammals “EDT”). 95% HPD uncertainty is displayed as background shade using model averaging by weighting each tree and parameter contributions by their posterior probability. The thickness of the line corresponds to the state-specific reconstructed number of lineages (in natural logarithm) that survived to the present. (H) Region-specific LTT aggregated from ancestral posterior probabilities of being tropical, extra-tropical, or widespread (SI Appendix). (I) LTT proportions in each state across time (lines correspond to the 10 pseudo-posterior trees).
Fig. 4.
Fig. 4.
Goodness of fit of diversification models. Prediction coverage from the richness probability distributions for diversification models, in increasing complexity: Space = geographic dependence (G), Time = environmental dependence (E), and Hidden = hidden states (H). Prediction coverage is defined as the model-predicted quantile of species richness using simulations that contains the empirical measurement of true species richness. For each clade (rows), for each model (column), a subrow consists of results for each of the 10 trees in sequence (subcolumns) and represents, in order, tropical, extra-tropical, and widespread richness and proportion of tropical/extra-tropical and widespread/extra-tropical.
Fig. 5.
Fig. 5.
Spatiotemporal diversification dynamics in mammal fossil record. Results from Cenozoic fossil biogeographic analyses at the genus level for tropics in green and extra-tropics in blue. Dashed horizontal lines reflect results from time-constant models while solid skylines show time-heterogeneous rates following stratigraphic periods (shading reflects 95% HPD intervals). Top: fossil extinction rates for the tropics (μT*) and the extra-tropics (μE*). Middle: fossil dispersal rates from the tropics to the extra-tropics (δT → E*) and vice-versa (δE → T*). Bottom: fossil sampling rates for the tropics (qT*) and the extra-tropics (qE*) (Mastodon silhouette credits: Becky Barnes’ North Dakota Geological Survey).
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