True seals achieved global distribution by breaking Bergmann's rule

Abstract

True seals (phocids) have achieved a global distribution by crossing the equator multiple times in their evolutionary history. This is remarkable, as warm tropical waters are regarded as a barrier to marine mammal dispersal and-following Bergmann's rule-may have limited crossings to small-bodied species only. Here, we show that ancestral phocids were medium sized and did not obviously follow Bergmann's rule. Instead, they ranged across a broad spectrum of environmental temperatures, without undergoing shifts in temperature- or size-related evolutionary rates following dispersals across the equator. We conclude that the tropics have not constrained phocid biogeography.

Keywords: Antitropical distribution; Bergmann's rule; Phocidae; biogeography; body size; sea surface temperature.

Figure 1

Biogeography of crown Phocidae. (a) Phylogeny of extant true seals (Rule et al. 2021b) with geographic distributions. (b) Dispersals for crown‐phocids (modified from Rule et al. 2020a), with the tropics in light red and polar environments in light blue. (c) Variation of Sea Surface Temperature by latitude through time (Herbert et al. 2016), demonstrating consistently high temperatures in the tropics, and broader Sea Surface Temperatures across latitudes closer to the present.

Figure 2

Evolution of body size in extant true seals estimated by RRphylo analysis, using phylogeny from Rule et al. (2021b). (a) Ancestral state estimation and (b) evolutionary rates for Log₁₀ minimum total body length. (c) Ancestral state estimation and (d) evolutionary rates for Log₁₀ maximum total body length. Timescales in millions of years.

Figure 3

Evolution of sea surface temperature (SST) in extant true seals estimated by RRphylo analysis, using phylogeny from Rule et al. (2021b). Ancestral state estimation (a) and evolutionary rates (b) for minimum SST. Ancestral state estimation (c) and evolutionary rates (d) for median SST. Ancestral state estimation (e) and evolutionary rates (f) for maximum SST. Timescales in millions of years.

Figure 4

Evolution of body size and SST in extant and extinct true seals estimated by RRphylo analysis, using phylogeny from Rule et al. (2021b). Ancestral state estimation (a) and evolutionary rates (b) for log₁₀ total body length. Ancestral state estimation (c) and evolutionary rates (d) for median SST. Timescales in millions of years.

Figure 5

Regression analyses for log total body length and median SST in extant and extinct true seals. (a) Linear regression of log₁₀ total body length versus median SST (adjusted R ² = 0.017, P‐value = 0.226). (b) Phylogenetic generalized least squares regression for log₁₀ total body length versus median SST (P‐value = 0.753).

Figure 6

Posterior distribution of the evolutionary rate matrices for the merged MCMC chains. Histograms show the posterior distribution of evolutionary rate variance values for log₁₀ total body length (TBL, top left) and sea surface temperature (SST, bottom right); and pairwise evolutionary covariance values between log total body length and sea surface temperature (top right). Ellipses (bottom left) are 50 bivariate distributions randomly sampled from the posterior distribution. The vertical orientation of the ellipses demonstrates that there is no evolutionary correlation between log₁₀ total body length and sea surface temperature. The elongated shape of the ellipses demonstrates that log₁₀ total body length has faster evolutionary rates than sea surface temperature.

Figure A1

Clade numbers for overfitRR analysis of all datasets in Table A7.

Figure A2

Log‐likelihood trace plot and acceptance ratio of first evolutionary rate matrix MCMC chain. MCMC chain ran for 1 million generations, with the first 25% discarded as burnin and sampling every 1000 generations. Acceptance ratio for the MCMC chain was ∼0.41 (correlation = 0.62; standard deviation = 0.15; root = 0.93).

Figure A3

The prior distribution of the first evolutionary rate matrix MCMC chain.

Figure A4

Log‐likelihood trace plot and acceptance ratio of the second evolutionary rate matrix MCMC chain. MCMC chain ran for 1 million generations, with the first 25% discarded as burnin and sampling every 1000 generations. Acceptance ratio for the MCMC chain was ∼0.41 (correlation = 0.62; standard deviation = 0.15; root = 0.93).

Figure A5

The prior distribution of the second evolutionary rate matrix MCMC chain.

Figure A6

Histogram of the posterior distribution of evolutionary correlation among log total body length and sea surface temperature, extracted from the two merged MCMC chains. Minimum = −0.68; 1st quartile = −0.03; Median = 0.09; Mean = 0.09; 3rd quartile = 0.21; Maximum = 0.79.

Figure A7

Posterior distribution of root values for log total body length (TBL) and sea surface temperature (SST) sampled from the merged MCMC chains.