Latitudinal gradients of biodiversity and macroevolutionary dynamics are prominent yet poorly understood.

• Abstract
• Model Development
• Results and Discussion
• Methods
• Acknowledgements
• References
• Figures

Biology Articles » Biodiversity » Kinetic effects of temperature on rates of genetic divergence and speciation » Methods

Methods

- Kinetic effects of temperature on rates of genetic divergence and speciation

Molecular Evolution Data. The SSU rDNA data in Fig. 1 were compiled from the sources cited in Appendix 1. Our model predicts that rates of molecular evolution increase exponentially with temperature (Eq. 3), which implies that the warmer, more rapidly evolving taxon makes a greater contribution to the genetic divergence, D, and hence to the calculated rate of molecular evolution f_o = D/2 (following Eq. 5), where is the time since divergence. To account for the greater contribution of the warmer-bodied taxon to f_o, we characterize the overall habitat temperature for each taxon pair depicted in Fig. 1 by using the Boltzmann average,

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, T_E, 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 V_m. (v) We repeated steps ii–iv 100 times to generate the 95% CIs for V_m 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, J_A. We estimated the temperature dependence of the per capita speciation rate, characterized by E (Eq. 8), and the normalization parameter, v_o, 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, f_oM^1/4e^E/kT, is equal to f_0ob_o. For primates, we obtain an estimate of 2.5 x 10¹³ J·g^–1·substitutions^–1·nucleotide for 1/f_0o by using an estimate of b_o 3.9 x 10⁸ W g^–3/4 for endotherms (17) and the geometric mean of the estimates of f_oM^1/4e^E/kT in ref. 14 for the globin gene (4.9 x 10¹⁰ substitutions·nucleotide^–1·10^–8 yr·g^1/4). For planktonic foraminifera, we obtain an estimate of 1.8 x 10¹³ J·g^–1·substitutions^–1·nucleotide for 1/f_0o by using an estimate of b_o 2.8 x 10⁷ W g^–3/4 for unicells (17) and the geometric mean of the estimates of f_oM^1/4e^E/kT for the data depicted in Fig. 1 (5.0 x 10⁹ substitutions·nucleotide^–1·10^–8 yr·g^1/4).