Abstract
Speciation is thought to be predominantly driven by the geographical separation of populations of the ancestral species. Yet, in the marine realm, there is substantial biological diversity despite a lack of pronounced geographical barriers. Here, we investigate this paradox by considering the biogeography of marine mammals: cetaceans (whales and dolphins) and pinnipeds (seals and sea lions). We test for associations between past evolutionary diversification and current geographical distributions, after accounting for the potential effects of current environmental conditions. In general, cetacean lineages are widely dispersed and show few signs of geographically driven speciation, albeit with some notable exceptions. Pinnipeds, by contrast, show a more mixed pattern, with true seals (phocids) tending to be dispersed, whereas eared seals (otariids) are more geographically clustered. Both cetaceans and pinnipeds show strong evidence for environmental clustering of their phylogenetic lineages in relation to factors such as sea temperature, the extent of sea ice, and nitrate concentrations. Overall, current marine mammal biogeography is not indicative of geographical speciation mechanisms, with environmental factors being more important determinants of current species distributions. However, geographical isolation appears to have played a role in some important taxa, with evidence from the fossil record showing good support for these cases.
Highlights
Marine mammals are an excellent case study for the role of geographically-driven diversification within open environment
Overall phylogenetic turnover is not associated with geographic distance (Fig. 1ai, partial r = 0.23, SES = -2.09, P = 0.028; see Supplementary File S1 for full results)
No species pairs returned a significant result for geographical clustering despite several pairs returning high empirical correlation values, e.g., Node 28 O. brevirostris + O. heinsohni, Node 20 Sousa chinensis + S. teuszii, Node 65 Hyperoodon ampullatus + H. planifrons
Summary
“Since the ecology of marine organisms is fundamentally different from that of such typical land animals as mammals, birds, land snails, and butterflies, one might expect modes of speciation in the oceans that differ completely from the typical geographic speciation of land animals.” (Mayr 1954). Tests for the role of geographical isolation within open environments are confounded by post-speciation, environmentally-driven range shifts. Such distributional changes could erase speciation-related geographical patterns (Losos and Glor 2003); given that diversification typically occurs across hundreds of thousands to millions of years (Hedges et al 2015), whereas environmental change can be far more rapid (Petit et al 1999). Cetaceans and pinnipeds provide a useful comparison because of their fundamentally different reproductive strategies, with the former being fully aquatic, while the latter must return to land or ice to breed Both allopatric divergence (Steeman et al 2009) and adaptive radiation (Slater et al 2010, Marx and Uhen 2010) have been proposed as primary drivers of extant cetacean diversity. A signature of geographical diversification within either group would suggest that dispersal limitations in the ocean can drive speciation
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