1 Environmental versus geological barriers in the Great American Biotic Interchange: a 2 biogeographic analysis using frogs 3 4 5 6 Carlos Alberto Jiménez Rivillas 7 Student 8 9 10 Andrew J. Crawford 11 Advisor 12 13 14 Catalina González Arango 15 Co-advisor 16 17 18 19 Universidad de los Andes 20 Departamento de Ciencias Biológicas 21 22 de Octubre de 2018 22 Bogotá D.C., Colombia 23 24 25 26 He [the naturalist] looks upon every species of animal and plant now living as the 27 individual letters which go to make up one of the volumes of our earth's history; and, as a 28 few lost letters may make a sentence unintelligible, so the extinction of the numerous 29 forms of life which the progress of cultivation invariably entails will necessarily render 30 obscure this invaluable record of the past. 31 Alfred Russel Wallace (1863) On the physical geography of the Malay Archipelago. The 32 Journal of the Royal Geographical Society of London 33:217-234. 33 34 35 36 I saw with regret, (and all scientific men have shared this feeling) that whilst the number of 37 accurate instruments was daily increasing, we were still ignorant 38 Alexander von Humboldt, Aimé Bonpland (1818) Personal Narrative of Travels to the 39 Equinoctial Regions of America, During the Year 1799-1804 - Volume 1. 40 41 42 43 44 45 Environmental versus geological barriers in the Great American Biotic Interchange: a 46 biogeographic analysis using frogs 47 48 Carlos Jiménez-Rivillas1,2, Paola Montoya3, Roberto Ibáñez4, Catalina Gonzalez-Arango5, 49 and Andrew J. Crawford2,4. 50 51 1M.Sc. student in Biological Sciences, 2Biom|ics Lab, Biological Sciences Department, 52 Universidad de los Andes, Bogota D.C. – Colombia. 53 3Laboratorio de Evolución de Vertebrados (EvolVert), Biological Sciences Department, 54 Universidad de los Andes, Bogota D.C. – Colombia. 55 4Smithsonian Tropical Research Institute (STRI), Panama. 56 5Laboratorio de Paleoecología y Palinología (PaleoLab), Biological Sciences Department, 57 Universidad de los Andes, Bogota D.C. – Colombia. 58 59 Abstract 60 The geological closure of the Isthmus of Panama (IP) precipitated one of the greatest 61 biogeographic events of the Cenozoic that indelibly changed composition of biotic 62 communities in South and North America. The precise timing of uplift and final closure of 63 the IP continues to be a topic of intense debate in geology and evolutionary biology. The 64 traditional or Young Isthmus model states that the definitive closure of the IP occurred 65 between 4 and 3 million years ago (Ma). The more recently proposed Old Isthmus model 66 states that the IP was completed during the middle Miocene (15 to 13 Ma). Regardless of 67 the closure data, the fossil record makes clear that at 2.7 Ma began the Great American 68 Biotic Interchange (GABI), a massive interchange of mammalian lineages, many affiliated 69 with dry and open environments. For the Old Isthmus hypothesis to be viable, one must 70 posit the existence of some non-oceanic barrier that delayed the interchange between 71 continents for some 10 million years. Here, we tested the hypothesis that an 72 environmental barrier in the form of a humid, closed-canopy forest was present on the IP 73 prior to the Pleistocene interchange and glacial cycling. Scant paleoenvironmental data 74 are available from the Neogene IP, so here we test our hypothesis indirectly using 75 comparative phylogeography of 69 species of anurans by reconstructing their 76 environmental affinities and estimating the timing of interchange between continents for 77 each lineage. We found that frog species with a preference for dry and open environments 78 all moved between continents after 3 Ma (n = 11 colonization events), while the mean 79 date of interchange for those associated with humid forests was 6.1 Ma (n = 24 events, SD 80 = 3.7 Ma), including some more recently than 3 Ma. Semi-arid species crossed 81 significantly later than humid forest species (randomization test, mean difference = 6.8 82 million years, P = 0.0075), as predicted by the hypothesis that a humid, closed-canopy 83 forest barrier existed during the Late Miocene and perhaps to a lesser extent in the 84 Pliocene. The Pleistocene was characterized by cooler and drier conditions which led to a 85 reduction in forest cover and this may have promoted directly the GABI across an IP 86 already 10 million years old. 87 88 Keywords: Crown age, dispersal, humid-closed barrier, Stem age, dry-open corridor 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 INTRODUCTION 112 113 Barriers to dispersal are an important component of vicariance biogeography and 114 models of allopatric speciation (Platnick & Nelson, 1978; Ronquist, 1997). Physical 115 barriers are often invoked as isolating mechanisms driving allopatric speciation and 116 creating regions of endemism (Rosen, 1988). Examples include oceans, rivers, and 117 mountains (Jansson, 2003). Environmental heterogeneity can also create barriers to 118 dispersal, however, such as the warm desert of North America (Hafner & Riddle, 2011). 119 Wet forest can also be a barrier to dispersal for organisms that prefer open or xeric habitat, 120 e.g., the Amazonian rain forest separates the xeric and open habitats of the Dry Diagonal 121 in the South from the coastal xeric habitats of Venezuela and Colombia in the North 122 (Gutiérrez et al., 2014). Here we use comparative phylogeography to ask whether closed- 123 canopy wet forest could have been a barrier preventing animals from dispersing through 124 the Isthmus of Panama (IP) during the Pliocene and Miocene. 125 126 Since the Early Cretaceous and throughout the Paleogene, South America lacked 127 connections with other major landmasses and was an island continent whose fauna and 128 flora remained in ‘splendid isolation’ (Simpson, 1980). Eventually the Central American 129 Seaway (CAS) closed via the formation of a complete and permanent land bridge known 130 as the Isthmus of Panama (IP) that joined South America with Central America, and thus 131 North America, separating the Caribbean Sea from the Pacific Ocean (Keigwin, 1978). The 132 date of the formation of the IP, however, is highly controversial and the scientific debate 133 itself has been termed the ‘Battle for the Americas’ (Stone, 2013). 134 135 Positions regarding the geological formation and completion of the IP fall under 136 two basic models. The traditional or ‘Young Isthmus’ model states that the IP was not 137 completely formed until the Pliocene approximately 3 million years ago (Ma). Early 138 evidence came from biostratigraphy and isotope paleogeography (Keigwin, 1978). 139 Subsequent geological and paleontological studies, along with molecular phylogenetic 140 studies of marine geminate species, also dated the closure of the IP to between 4.0 to 3.0 141 Ma (e.g., Coates et al., 1992; O’Dea et al., 2016; Coppard & Lessios, 2017). These young 142 dates for the closure of the IP contrast sharply with the more recent ‘Old Isthmus’ model 143 that posits a closure around 15 to 13 Ma, based on analyses of petrogenesis of magmatic 144 rocks and paleo-volcanic activity (Montes et al., 2015). 145 146 While controversy surrounds the estimated age of the closure of the IP, the date of 147 the Great American Biotic Interchange (GABI) is very well established. ‘GABI’ in the strict 148 sense refers to the massive, nearly simultaneous, and reciprocal colonization of North and 149 South America by diverse mammalian lineages at 2.7 Ma, a date clearly established by an 150 extensive fossil record involving species from 17 taxonomic families (Webb, 1976; 151 Marshall et al., 1982). The date of the GABI fits well with the Young Isthmus model, 152 whereas the Old Isthmus model posits a land bridge being completed at least 10 million 153 years before the GABI took place. To reconcile the well-dated GABI with the Old Isthmus 154 model, proponents of the latter would have to explain why a completed terrestrial corridor 155 was not used until 10 million years later. 156 157 A hypothesis recently put forward to explain this 10 million year lag time invokes 158 an environmental barrier (Bacon et al., 2016). While a land bridge may have been in 159 place by the middle Miocene, successful dispersal would not take place if organisms 160 encounter conditions very different from their usual environment. The mammals 161 participating in GABI included a large proportion of species specializing in savannah 162 habitat (Stehli & Webb, 1985; Webb, 1991; Vrba, 1992; Woodburne, 2010). Thus, if a 163 complete, late Miocene Isthmus were covered predominately by closed-canopy wet 164 forests, such a landscape could have acted as an environmental, rather than physical, 165 barrier to dispersal between continents, delaying the GABI despite the existence of a 166 complete land bridge (Bacon et al., 2016). 167 168 Unfortunately, details are lacking on the spatial and temporal distribution of paleo- 169 environments such as open and semi-arid versus closed and forested habitats during the 170 late Miocene, and the habitat preferences of many species that participated in the GABI 171 remain unconfirmed (Bacon et al., 2016). In this study, we proposed using comparative 172 phylogeography from non-mammalian groups to test for an association between habitat 173 preference and timing of continental interchange. Frogs may provide a valuable proxy for 174 the historical presence of open or wet versus closed or semi-arid habitats. As the only non- 175 amniote tetrapods, amphibians tend to be found close to source of water and most species 176 are poor dispersers (Cushman, 2006). These characteristics of frogs make them excellent 177 indicators of biogeographical processes (Ron, 2000; Paz et al., 2015).