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The historical biogeography of the Northern and Southern hemispheres has long been of interest to biologists (Lomolino et al., 2010). Since its inception, continental drift theory has provided exemplar systems in support of vicariance biogeography (Nelson & Platnick, 1981; Morrone & Crisci, 1995). However, the importance of vicariance in biogeography has been reconsidered in the last decade, as evidence for dispersal has accumulated from increasing numbers of molecular phylogenetic studies (e.g. Nothofagus spp.; Cook & Crisp, 2005). Indeed, numerous studies have revealed that vicariance is not always the dominant factor explaining patterns of distribution in widespread groups of organisms (e.g. Sanmartín et al., 2001; Sanmartín & Ronquist, 2004; Warren et al., 2010). Consequently, historical biogeography now tends to systematically investigate the relative roles of dispersal and vicariance events to better understand global patterns of biodiversity distribution by the integration of molecular dating approaches (Ree & Smith, 2008; Lomolino et al., 2010).
To explain the biogeographical history of the Northern Hemisphere, two major colonization routes have been hypothesized: (1) the North Atlantic land bridge (NALB) and (2) the Bering land bridge (BLB) (Tiffney, 1985; Milne, 2006; Fig. 1). The NALB is thought to have been used most during the early Cenozoic, connecting the biotas of North America and southern Europe via Greenland by a northern De Geer route, or by a southern Thulean route via Iceland and Great Britain (Tiffney, 1985; Fig. 1). The southernmost connection was severed during the early Eocene, approximately 50 million years ago (Ma), when the climate was mostly subtropical at these latitudes (Sluijs et al., 2006). The northernmost connection persisted until 40 Ma (Milne, 2006). The connection between Asia and North America via the BLB remained throughout most of the Cenozoic and was only severed at approximately 5.5–4.8 Ma (Marincovich & Gladenkov, 1999; Milne, 2006; Fig. 1). It is believed that the BLB was most important for inter-continental colonizations after the NALB disappeared (Tiffney, 1985; Tiffney & Manchester, 2001). The BLB reappeared during the Quaternary glacial epochs, when migration across the BLB was only possible for taxa of arctic or boreal affinities (Milne, 2006).
Figure 1. Cenozoic biogeographical hypotheses for the Northern Hemisphere (red), the Southern Hemisphere (blue) and interchanges between the two hemispheres (green). Three time slices are mapped onto palaeogeographical reconstructions (redrawn from Blakey, 2008). For each period, the palaeoclimatic envelopes (arid, cold, cool temperate, warm temperate, paratropical and tropical) have been latitudinally and longitudinally delimited. For each time slice, arrows represent the putative colonization routes. Darker colours indicate higher rates of dispersal and lighter colours indicate lower rates. Some important geological features that could have acted as barriers to faunal dispersal (such as the Turgai Sea) are shown. BLB, Beringian land bridge; DGR, De Geer route; TR, Thulean route; TuR, Turgai route.
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The Southern Hemisphere provides a prime example of a vicariance scenario, with disjunct trans-Pacific or trans-Indian distributions resulting from the sequential breakup of the southern supercontinent Gondwana starting 180 Ma (Lomolino et al., 2010). Biogeographical consequences of southern plate tectonics are manifold, most of them stemming from the effect of plate movements on the configuration of landmasses and marine basins, which then determine degrees of biotic isolation and opportunities for biotic exchange (Lomolino et al., 2010; Fig. 1) or more indirectly influence regional and global climates (Lomolino et al., 2010; Fig. 1). Thus, numerous dispersal routes have been revealed between continents (Sanmartín & Ronquist, 2004; Fig. 1). Several landmasses separated about 130–100 Ma, with South America drifting westwards from Africa as the Atlantic Ocean opened, and the Madagascar block breaking off from India (Blakey, 2008). Ancient groups generally harbour a disjunct biogeographical distribution caused by the creation of oceanic barriers to biotic dispersal (Lomolino et al., 2010). The continent of Australia–New Guinea began to gradually separate from Antarctica 80 Ma and actively moved north (55 Ma), but retained some connection with the remainder of Gondwana for about 10 million years (Beu et al., 1997; Sanmartín & Ronquist, 2004). After being separated from Madagascar (90 Ma), the Indian Plate collided with Asia about 45 Ma, forming the Himalayas and modifying climate as well as plate motion in Southeast Asia (Hall, 2002; Blakey, 2008; Fig. 1). At the same time, the southernmost part of Australia finally separated from Antarctica, closing the direct route to South America. During the Oligocene, South America finally separated from west Antarctica (Blakey, 2008). Afterwards, South America and Antarctica were not directly connected, but the multiple microplates of the Antarctic Peninsula remained near southern South America, acting as stepping stones and allowing biological interchange by the Drake Passage until its complete opening 23 Ma (Beu et al., 1997; Fig. 1). About 20 Ma, the collision between the Australian Plate and the south-western part of the Pacific Plate created new routes for biotic exchange with Southeast Asia (Hall, 2002; Fig. 1), also causing major climate changes that drastically modified weather patterns in Australasia. Finally, South America was connected to North America via the Isthmus of Panama, thereby allowing the Great American Faunal Interchange (3.5 Ma; Lomolino et al., 2010; Fig. 1).
Based on their different geological histories, several major differences are evident in biogeographical events between the Northern and Southern hemispheres. In the Northern Hemisphere, complex biogeographical histories involving old vicariance events or recent dispersals are generally recovered for plants and terrestrial animals (Sanmartín et al., 2001). By contrast, in the Southern Hemisphere, animal distributions are more congruent with the fragmentation of Gondwana whereas plant distributions are generally better explained by dispersal events (Sanmartín & Ronquist, 2004).
It is reasonable to expect that the existing distribution of biodiversity has been deeply influenced by the complex geological history of both hemispheres. To better understand the precise contribution of past geological events in shaping global biodiversity patterns, inference of the historical biogeography of cosmopolitan groups is essential. In this study, we use swallowtail butterflies (Lepidoptera, Papilionidae) as a model system to assess the role played by vicariance or colonization routes in shaping distribution patterns. Swallowtails are particularly suitable for testing the contribution of these processes because several groups exhibit disjunct distribution patterns in both hemispheres (Tyler et al., 1994; Scriber et al., 1995). Overall, the family comprises 32 genera and about 550 described species (Häuser et al., 2005), which are usually classified into three subfamilies: Papilioninae, Parnassiinae and Baroniinae (Simonsen et al., 2011). The subfamily Papilioninae is the largest with more than 485 species world-wide, reaching its greatest diversity in the tropics (Wallace, 1865; Zakharov et al., 2004). This subfamily harbours a disjunct distribution and is traditionally considered to exemplify vicariance (plate tectonic breakup of Gondwana), particularly within the Troidini (Braby et al., 2005) and the genus Papilio (Zakharov et al., 2004). The subfamily Parnassiinae is mainly distributed in the Palaearctic, with 67 species mostly occurring in central Asia and some specialized groups in high-elevation habitats (Weiss, 1991; Nazari et al., 2007; Michel et al., 2008). Several Parnassius species also occur in the Western Nearctic, a distribution considered to result from recent dispersal events (Michel et al., 2008; Todisco et al., 2012). Finally, the sole member of the subfamily Baroniinae, Baronia brevicornis, is endemic to Mexico and is considered a relict taxon (Tyler et al., 1994; Scriber et al., 1995).
To advance our understanding of the biogeographical history of a group, a robust and well-resolved phylogenetic tree is required (Ree & Smith, 2008). Taxon sampling is thus fundamental, as sparse taxon sampling may bias phylogenetic relationships as well as influencing the reliability of any estimates of ancestral areas of distribution (Nylander et al., 2008). Phylogeny should also be well connected with a temporal framework to allow relevant comparisons with known past geological events (e.g. changes in sea level, continental drift, orogenesis; Wiens & Donoghue, 2004; Lomolino et al., 2010). Although earlier studies have suggested an old origin – Late Jurassic – for swallowtails (e.g. Zeuner, 1943; Miller & Miller, 1997; Braby et al., 2005), most DNA-based studies have inferred a more recent origin between the Late Cretaceous and the early Cenozoic for various swallowtail groups (e.g. Zakharov et al., 2004; Nazari et al., 2007) as well as for the whole family (Simonsen et al., 2011; Condamine et al., 2012a). The more recent origin for swallowtails is not consistent with the hypothesis of a Gondwanan origin (Parsons, 1996a,b; Miller & Miller, 1997; Braby et al., 2005). If the diversification of swallowtails occurred during the Cenozoic, dispersal may be the main biogeographical process responsible for their present distribution. In this context it is essential to identify the different dispersal routes that the most recent common ancestors (MRCAs) of extant papilionid species may have used to colonize all continents in both hemispheres. Using comprehensive taxon sampling and two distinct biogeographical approaches, we specifically focus on assessing the timing and nature of these biogeographical events to investigate the use of known dispersal routes in the Northern and Southern hemispheres (Sanmartín et al., 2001; Sanmartín & Ronquist, 2004).