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Latitudinal gradients of biodiversity and macroevolutionary dynamics are prominent yet poorly understood.
where T1 and T2 are the habitat temperatures of the two taxa in Kelvins. Habitat temperatures were independently estimated by using a global compilation of contemporary community abundance data collected from 1,265 sites around the world (44) in conjunction with contemporary ocean temperature data (45). Habitat temperatures were estimated by using sea-surface temperatures for shallow-dwelling taxa and temperatures at 200-m depth for deeper-dwelling taxa (Appendix 1).
Genetic Divergence Data. The SSU rDNA data in Fig. 2 were compiled from the sources cited in Appendix 2. The habitat temperature of each population was estimated from the spatial location of sampling by using contemporary ocean temperature data (45). The Boltzmann-averaged habitat temperature, TE, was then calculated for each taxon pair depicted in the figure.
FO Data. The latitudinal distribution of FOs of morphospecies in Fig. 3B was analyzed by using morphospecies-level data in the Neptune database, a compilation of fossil samples from over 160 deep-sea drilling holes around the world that have been dated to an average precision of 32). We analyzed the Neptune data by using the following procedure to simultaneously control for latitudinal variation in area (Fig. 3A) and for the effects of sampling effort on paleontological analyses (46): (i) We assigned each of >3,000 core samples to one of four latitudinal bands of equal ocean surface area (Fig. 3A) and to one of six 5-Ma time intervals spanning the last 30 Ma. (ii) We selected a subset of 40 samples at random and without replacement from each equal-area latitudinal band and time interval, yielding a data subset comprising >900 samples. (iii) We determined the band of FO for each morphospecies of foraminifera arising through speciation over the past 30 Ma. (iv) We tallied the total number of FOs in each band to obtain estimates for Vm. (v) We repeated steps ii–iv 100 times to generate the 95% CIs for Vm depicted in Fig. 3B (Appendix 3).
Paleotemperature Data. To obtain the estimates of average ocean temperature depicted in Fig. 3B, , we modeled variation in sea-surface temperatures with respect to latitude, L (–90° to 90°N), and time, t, by using the heat equation on the surface of a sphere, T(L, t) = (P(t) – T0)sin2(L/180) + T0, where P(t) is the sea-surface temperature at the poles at time t, and T0 is the sea-surface temperature at the equator. The function P(t) was estimated in Fig. 2 of ref. 33 by using robust methods of paleotemperature calibration. The parameter T0 was assumed to remain constant at 28°C over the past 30 Ma based on available evidence (47). The function T(L, t) was integrated over time and space, as described in Appendix 4, to yield the estimates of depicted in Fig. 3B.
Estimating the per Capita Speciation Rate. Evaluating the temperature dependence of the per capita speciation rate (Eq. 8) required explicitly accounting for temperature-dependent changes in foraminifera community abundance across latitudes. To avoid difficulties associated with inferring live abundances of foraminifera from shell accumulation rates, we characterized this temperature dependence by using a global compilation of plankton tow data (45) on foraminifer metacommunity abundance per unit area, JA. We estimated the temperature dependence of the per capita speciation rate, characterized by E (Eq. 8), and the normalization parameter, vo, by expressing the latitudinal distribution of FOs as a cumulative function of ocean area (Fig. 3A), paleotemperature T(L, t), and metacommunity abundance (Appendix 5).
Estimating the Energy Required to Induce Mutations. Following Eqs. 2–5, the size- and temperature-corrected rate of molecular evolution, foM1/4eE/kT, is equal to f0obo. For primates, we obtain an estimate of 2.5 x 1013 J·g–1·substitutions–1·nucleotide for 1/f0o by using an estimate of bo 3.9 x 108 W g–3/4 for endotherms (17) and the geometric mean of the estimates of foM1/4eE/kT in ref. 14 for the globin gene (4.9 x 1010 substitutions·nucleotide–1·10–8 yr·g1/4). For planktonic foraminifera, we obtain an estimate of 1.8 x 1013 J·g–1·substitutions–1·nucleotide for 1/f0o by using an estimate of bo 2.8 x 107 W g–3/4 for unicells (17) and the geometric mean of the estimates of foM1/4eE/kT for the data depicted in Fig. 1 (5.0 x 109 substitutions·nucleotide–1·10–8 yr·g1/4).
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