Panbiogeography, its critics, and the case of the
Transcription
Panbiogeography, its critics, and the case of the
CSIRO PUBLISHING Australian Systematic Botany, 2014, 27, 241–256 http://dx.doi.org/10.1071/SB14027 Viewpoint Panbiogeography, its critics, and the case of the ratite birds Michael Heads Buffalo Museum of Science, 1020 Humboldt Parkway, Buffalo, NY 14211-1293, USA. Email: [email protected] Abstract. Panbiogeographic analysis is now used by many authors, but it has been criticised in recent reviews, with some critics even suggesting that studies using the method should not be accepted for publication. The critics have argued that panbiogeography is creationist, that it rejects dispersal, that its analyses are disingenuous, and that it deliberately ignores or misrepresents key evidence. These claims are examined here, and are all shown to be without foundation. The distributions of the molecular clades of ratites have not been mapped before, and they are considered here in some more detail as a case study illustrating panbiogeographic methodology. Received 24 August 2014, accepted 10 October 2014, published online 31 March 2015 Introduction (1) ‘Panbiogeography is creationist’ Panbiogeographic methods for investigating distribution and evolution are now used widely, especially in Latin America. Recent publications include Moreira et al. (2011), MayaMartínez et al. (2011), Rosas et al. (2011), Pires and Marinoni (2011), Ribeiro and Eterovic (2011), Climo and Mahlfeld (2011), Grehan (2011), Arzamendia and Giraudo (2012), Mayén-Estrada and Aguilar-Aguilar (2012), Velásquez et al. (2012), Ferretti et al. (2012), Heads (2012a, 2012b, 2012c, 2014), CamposSoldini et al. (2013), Echeverry and Morrone (2013), Gallo et al. (2013), Goldani (2013, 2014) and Quijano-Abril et al. (2014). Although the methods and applications have drawn positive reviews (e.g. Nelson 2012; James 2013), several recent papers have criticised the methodology. Waters et al. (2013a) wrote: ‘. . .to convey our concerns that some mainstream evolutionary journals continue to publish papers that, in our view, present misleading accounts of biological evolution. Specifically, we argue that ‘panbiogeographic’ studies of spatiotemporal biological history . . . are detrimental to the progress of biogeography. . .’. The authors criticised the ‘editorial and review processes [that] continue to allow this misleading approach to be promulgated. . .’ (p. 494). The present paper examines whether or not there is any substance to the criticism. Waters et al. (2013a) cited the classic case of ratite biogeography as an example illustrating the problems with panbiogeographic methods, and so this particular group is examined in some more detail. In debates about evolutionary biology, authors often assert that their opponents are creationists. For example, Nelson and Platnick (1984) argued that modern systematics has moved beyond Darwinism; Dawkins (1986, p. 284), seeing an attack on Darwinism as an attack on evolution itself, responded that Nelson and Platnick ‘have earned themselves a place of honour in the fundamentalist, creationist literature’. (Other examples are given by Livingstone 2013.) McGlone (2005) wrote that panbiogeography supports ‘something close to special creation’, because it accepts a mode of speciation (vicariance) that does not involve dispersal. Waters (2005) suggested that including a discussion of panbiogeography in a text book (Crisci et al. 2003) was ‘akin to presenting creationism and evolution as two ‘equal’ scientific disciplines. . .’. Waters did not describe how panbiogeography resembles creationism, but O’Grady et al. (2012) did. They argued that panbiogeographers are ‘like creationists’ as their approach: Criticisms of panbiogeographic analysis Recent critics have proposed four main arguments, namely, that (1) panbiogeography is creationist, that (2) it rejects dispersal, that (3) its analyses are disingenuous and (4) that it ignores or misrepresents key evidence (Holland 2012; O’Grady et al. 2012; Waters et al. 2013a). These four claims are examined next. Journal compilation CSIRO 2014 . . .is analogous to the wedge strategy proposed by creationists and advocates of intelligent design. First, a case is made that dogmatic thinking has overtaken good science due to researchers viewing their data only with a single lens, eliminating hypotheses because of institutionalized biases within the field. Then, a ‘theory’ is presented as a reasonable alternative on equal footing with the current dogma. [O’Grady et al. 2012, p. 703] Yet this is the strategy used in many scientific studies; a typical enquiry establishes that a standard model does not account for observed phenomena and, on the basis of the evidence, an alternative explanation is proposed. Waters et al. (2013a) wrote ‘when panbiogeographic hypotheses of ancient vicariance conflict with data from geology, palaeontology and molecular genetics (as they almost www.publish.csiro.au/journals/asb 242 Australian Systematic Botany inevitably do), panbiogeographers tend to dismiss these other information sources as unreliable’. The authors did not cite any actual data that do conflict (there are none), but the idea that there is a conflict provides a basis for their conclusion: ‘this attitude is akin to a young-earth creationist insisting that the world was created in 4004 BC. . .’. The reason for this heightened rhetoric is not because panbiogeographic results conflict with the primary data, but because they challenge the traditional interpretations of the Modern Synthesis, as supported by Waters et al. (2013a). Holland (2012) compared panbiogeography with ‘other efforts aimed at undermining confidence in empirical science, namely the creationist/intelligent design and climate change denial movements. Similarities in approach include attempts to cast doubt, exaggerate controversy and overplay scientific uncertainty where convenient, in support of the agenda’. Thus, any biologist who ‘casts doubt’ on orthodox theory is against empirical science and resembles a creationist. This perspective can be contrasted with the idea that doubt is a good thing in science, as supported by authors such as Canetti (1962), who wrote that ‘a scholar’s strength consists in concentrating all doubt onto his special subject’. Rational scepticism does not undermine confidence in empirical science, it is the basis of it and the reason for it. Thus, the motto of the Royal Society is ‘Nullius in verba’, that is, ‘take nobody’s word for it’. (2) ‘Panbiogeography does not accept that plants and animals disperse’ Holland (2012) suggested that ‘panbiogeographers strongly reject the significance of dispersal a priori, in any form. . .’ and that they ‘vehemently deny, a priori, any role of dispersal in natural history’. Likewise, O’Grady et al. (2012) wrote that panbiogeography involves ‘biased approaches that reject dispersal a priori’ and Waters et al. (2013a) agreed, using the same phrase verbatim. However, even young children know that animals and plants disperse, and the suggestion that panbiogeography rejects dispersal is simply not true; it is a strawman created by the critics. Waters et al. (2013a) wrote that ‘we have yet to see an empirical panbiogeographic study that argues for anything other than the primacy of some ancient vicariant process to explain distributional data’ (p. 495). O’Grady et al. (2012) proposed that ‘a central tenet of Heads’s (2012a) perspective on biogeography is that most patterns observed can be explained by the breakup or intercalary extinction of widespread metapopulations’. These claims are not correct though, and vicariance explains only allopatry, not distributional overlap. If vicariance were the only process, every area on Earth would have only one endemic clade. In fact, of course, many groups have overlapping distributions, and in panbiogography, this is attributed to dispersal (rather than sympatric speciation). For example, the widespread overlap of primate clades by range expansion (dispersal) in America, Africa and Asia, is discussed at length in the book (Heads 2012a) that O’Grady et al. (2012) and Waters et al. (2013a) both cited. Although panbiogeography does not reject dispersal, it does question its significance in establishing distribution in particular cases. For example, Waters et al. (2013a) wrote that M. Heads ‘panbiogeographers routinely scorn the role of long-distance dispersal in assembling oceanic island biotas. . .’ (p. 495), but inferring unlikely events of long-distance dispersal from distant continents to a young island is unnecessary if former islands existed in its vicinity. Waters et al. (2013a) wrote as follows: ‘unfortunately, this a priori rejection of long-distance dispersal [by panbiogeography] . . . ignores abundant evidence supporting this important process’ (p. 494). As an example of work ignored by panbiogeography, they cited Ali and Huber’s (2010) explanation for the presence of lemurs on Madagascar. Yet panbiogeography has not ignored this study; it was critiqued in a work that Waters et al. (2013a) cited (Heads 2012a, pp. 117, 118). All (or almost all) individual organisms have dispersed to their present location. In addition, many clades have expanded their range by dispersal, and this accounts for overlapping distributions. These ideas are more or less uncontroversial, although Darwin (1859) extrapolated further and assumed that all clades have attained their range by expanding outwards from a ‘centre of origin’. Clements (1909, p. 145) noted that Darwin’s assumption that species evolved at one spot ‘seems to be little more than inheritance from the special creationists’, and Gadow (1909, p. 326) made a similar observation. Nevertheless, the Modern Synthesis adopted the centre-oforigin model. Mayr (1965) concluded as follows: ‘quite obviously, except for a few extreme [local] endemics, every species is a colonizer because it would not have the range it has, if it had not spread there by range expansion, by ‘colonization’, from some original place of origin’. In a later discussion of speciation, Mayr (1982) wrote that although some textbooks showed ‘a widespread species cut in half by a geographical barrier’, ‘more detailed studies . . . suggest a different solution’, namely speciation by founder dispersal. Stebbins (1966, fig. 5-1) agreed with Mayr and showed allopatric differentiation developing solely by migration and ecological differentiation; there was no mention of the appearance of new geographic barriers. In other widely used text books on speciation, Grant (1971, 1981, 1985) included many maps of allopatry but, like Stebbins, did not mention geographic change as a cause of allopatric speciation. This assumption of a centre of origin and dispersal is supported by many ecologists (for example, Levin 2000) and palaeontologists (Eldredge et al. 2005); however, it is this a priori acceptance of the assumption that panbiogeography rejects. Following the early work in panbiogeography, the importance of vicariance is now generally acknowledged (in 1973 the term was cited 10 times, and in 2013 it was cited 1850 times; Google Scholar, accessed 20 July 2014). (3) ‘Panbiogeography is disingenuous’ A panbiogeographic treatment of the Hawaiian Islands cited former high islands in the Pacific that are now submerged, and mapped the 2000-, 4000- and 5000-m isobaths of the central Pacific (Heads 2012a, figs 7-1, 7-2). In response, Holland (2012) wrote that ‘the figures appear to be a disingenuous and misleading depiction aimed at advancing the vicariant agenda’. O’Grady et al. (2012) agreed that the figures were ‘more than Panbiogeography, its critics, and the case of the ratite birds slightly disingenuous’, because sea level has not dropped by more than ~100 m and so the submerged seamounts could not have been emergent. Yet the authors overlooked the thousands of metres of subsidence that the Pacific seafloor itself has undergone through the Cenozoic (van der Pluijm and Marshak 2004, p. 404; Hillier and Watts 2005; Zhong et al. 2007, fig. 1). As the seafloor has drifted away from the spreading ridge that produced it, namely the East Pacific Rise, it has cooled (increasing its density) over tens of millions of years, and it has subsided by these large amounts. This has led to the submergence of most of the earlier islands that had developed on it; the current high islands are new ones. Some of the most obvious evidence for subsidence is seen in the numerous atolls of the region. These are constructed from coral reefs that have grown on former islands as the seafloor and the islands subsided. In addition, the many flat-topped seamounts (guyots) in the Pacific, for example, at the north-western end of the Hawaiian–Emperor chain, are former high islands that were eroded to sea level before being submerged with the tectonic subsidence of the seafloor. Guyots also occur north, south and west of the Hawaiian–Emperor chain (Etnoyer et al. 2010; Heads 2012a; Gardner et al. 2013). All fresh lava on Hawaii has been colonised by organisms from somewhere else, but not necessarily from the nearest continent, thousands of miles away. In Heads (2012a), I listed several unjustified assumptions that have been made about Hawaiian biogeography. The first was the flawed reasoning that ‘the Hawaiian Islands have never been connected to other land masses and so must have received all their biota by long distance dispersal from the continents (mainly Asia and America)’ (Heads 2012a, p. 314). Although the first part of the sentence is correct, the second does not follow from it if there were prior islands closer to Hawaii than the continents. In their response to what I wrote, O’Grady et al. (2012) commented that ‘one cited ‘assumption’ is that the Hawaiian Islands have never been connected to other land masses, despite all the data supporting these islands as a young, volcanic hotspot archipelago’. This is disingenuous, because I never suggested that the Hawaiian Islands have been connected with other land masses. (4) ‘Panbiogeography ignores or misrepresents key evidence’ Waters et al. (2013a) suggested that panbiogeography ‘simply ignores the [past 25 years] of scientific progress in evolutionary theory, molecular genetics, computational biology and geology’ (p. 496). Yet, panbiogeogaphic studies have discussed many papers on all these topics (more than 2000, most published in the past decade, were referred to in Heads 2012a, 2014 alone). For example, with respect to geology, panbiogeography has considered the biological significance of guyots in the Musicians Seamounts north of Hawaii, the boundary between the accreted terranes and the craton in northern South America, the Central African Rift System (not the Great Rift), the terranes of Borneo, New Caledonia and New Zealand, uplift and subsidence in Fiji, strike-slip displacement on major faults in New Zealand and New Caledonia, and many other features, none of which had previously been cited by biogeographers. Australian Systematic Botany 243 Waters et al. (2013a) also wrote that ‘panbiogeographers have proposed scenarios that seemingly dismiss all other data regarding the history of life on Earth. . .’ (p. 495). Panbiogeographic work has rejected certain interpretations and hypotheses, but not data. The method relies on a wide range of recent data from biogeography, phylogenetics, palaeontology and tectonics. The critics disagree, not because we failed to refer to the molecular and geological data, but because we gave a novel interpretation of the data that they had not considered. The strange assertion that ‘The panbiogeographic approach usually ignores long-distance dispersal’ (Waters et al. 2013a, p. 494) again overlooks the detailed critiques of all aspects of the topic made by panbiogeographers over the past 50 years. Biogeography and fossils: the problem of the priors Holland (2012) argued that panbiogeography discounts the use of fossil ages to estimate lineage ages; however, this is not correct. Whenever they are available, fossils and their ages are incorporated in panbiogeographic analyses of particular groups (e.g. Heads 2012a). What the method does reject is the idea that maximum possible clade ages can be ascertained by a literal reading of the fossil record. Instead, the age of the oldest known fossil of a group is taken to represent a minimum age for its clade. As Smith (2007, p. 231) wrote, ‘the fossil record provides direct evidence but it cannot be taken at face value’. The same principle applies to fossil-calibrated clock studies. Waters et al. (2013a) wrote that ‘practitioners of the panbiogeographic approach routinely deny the utility of molecular-clock dating, presumably because molecular date estimates are usually much younger than those expected under vicariance’ (p. 496). Panbiogeography rejects the treatment of fossil-calibrated molecular clock dates as maximum (absolute) clade ages, and instead it interprets them as minimum ages. The technical problems involved in treating fossil-calibrated clade ages as maximum ages are explained in more detail in a paper on Bayesian dating (Heads 2012b). (Although this was published 3 months before Waters et al. (2013a) submitted their paper, it was overlooked in their critique). The key question is: just how much older than its oldest fossil can a clade be? In Bayesian studies, authors stipulate as ‘priors’, before any analysis, that the maximum age of a group can be no more than an arbitrary number of years older than the oldest fossil. The authors are free to choose any number, but in practice, the number chosen is usually very small, reflecting the traditional, illogical idea that a group is only a little older than its oldest fossil. The priors mean that young ages are calculated for the clades. This, in turn, means that vicariance can be ruled out and the traditional model of chance dispersal supported. Although the method is very popular, the ‘problem of the priors’ is now being acknowledged by a growing number of authors (see papers cited in Heads 2012b). For example, a recent discussion of the problem concluded that sporadic preservation in the fossil record ‘may be effectively impossible to model’ and that priors ‘should . . . be regarded as highly arbitrary’; their value ‘is something we simply cannot know’ (Pirie and Doyle 2012, pp. 108, 109). 244 Australian Systematic Botany M. Heads Central Pacific biogeography According to Holland (2012), panbiogeography ‘suggests that arcs of once-emergent ancient seamounts between, say, the Hawaiian and Marquesan islands played a role, by harboring the lineages we see today . . . over tens of millions of years. But again this would require numerous departures from what is understood and accepted by geophysicists’. In fact, this model would not involve any departures from standard geology, because it is well known that the many atolls and seamounts between French Polynesia and Hawaii (the Line Islands) are former high islands, produced during multiple phases of volcanism (but not, despite Holland’s (2012) suggestion, as arcs). Earlier dispersalists recognised this; Zimmerman (1948, p. 127) wrote that ‘many of the peculiar endemic groups of the Hawaiian and south-eastern Polynesian islands owe their existence, if not their very origin, to ancient high islands of the one-time splendid archipelagos now marked by clusters of coral reefs [or submerged seamounts]. . . Atolls have been overlooked, generally’. Sixty years later, the atolls and seamounts that surround the Hawaiian Islands are still neglected by dispersal theorists. Where they are acknowledged, the former islands that they represent are regarded merely as stepping stones that may have facilitated dispersal from the Pacific margins to the Hawaiian Islands (e.g. Mayr 1982, p. 453). However, former islands may have had a more fundamental role than this, because the different generations of islands could have allowed lineages to persist in the region since the Jurassic, at least. Waters et al. (2013a) suggested that instead of accepting long-distance dispersal, panbiogeography invokes ‘continental land bridges and island arcs of little or no geological credibility (see critique by O’Grady et al. 2012)’ (p. 495). This is a serious charge, but neither Waters et al. (2013a) nor O’Grady et al. 2012 provided a single actual example; this is because there is none. Again, the authors are attacking strawman arguments that panbiogeography has never advocated, to avoid confronting real issues such as the significance of the seamounts, the relevance of slab rollback for vicariance, the problem of the dating priors (see above), and many others. O’Grady et al. (2012) suggested that ‘a plethora of data, such as the well understood geological processes involved in the formation of the Hawaiian Archipelago . . . are ignored [by Heads 2012a]’. In fact, Hawaiian geology is controversial. Instead of ignoring the problems though, I cited many new studies on the topic that have not been mentioned before in biological work, and I discussed their implications. The underlying causes of Hawaiian volcanism are a topic of intense current debate. For example, is the proposed hotspot beneath Hawaii hotter than the surrounding mantle? Earlier work (cited in Heads 2012a) suggested that it is not, and a subsequent study by Presnall and Gudfinnsson (2011) also concluded that ‘Hawaii is not a hot plume. Instead, it shows magmas characteristic of normal mature-ocean thermal conditions in the low-velocity zone. We find no evidence of anomalously high temperatures of magma extraction and no role for hot mantle plumes. . .’. Presnall and Gudfinnsson (2011) developed a model for the Hawaiian volcanism that involves fracturing of the seismic lithosphere, not mantle plumes. Hamilton (2011) concluded that ‘all published tomographic models of purported deep plumes are severely According to Holland (2012), panbiogeography accepts that ‘there were emergent Pacific land masses, some of which connected the far-flung island archipelagos and continents surrounding the Pacific basin. . .’. This is not correct; for the central Pacific, panbiogeography accepts a complex history of volcanic islands and oceanic plateaus that have undergone subsidence, uplift, rifting, horizontal translation and accretion. It does not accept land masses connecting far-flung archipelagos. Panbiogeography adopts a metapopulation model, in which a widespread ancestor survives in situ, which may be a large region of the central Pacific, by colonising individually ephemeral islands as these become available. Colonisation is by whole populations using normal means of dispersal, not by individuals undergoing rare founder events that lead to speciation. Thus, the same metapopulation model that applies to groups occupying habitat ‘islands’ on continents also applies to populations on real islands in an ocean. In contrast, dispersal theorists reject a metapopulation model and local dispersal among islands, but at the same time accept founder dispersal from continents thousands of kilometres away. Many authors have suggested that ‘vicariance cannot explain endemic species on oceanic islands. . .’ (Coyne and Orr 2004, p. 124), but this is not correct. Migration of subduction zones and their associated island arcs away from a continent or from other islands (by slab rollback) is one important mechanism, rifting of active island arcs is another. The Solomon Islands–Vanuatu–Fiji–Tonga arc has undergone both processes, migrating away from the Gondwana mainland in the Cretaceous, and then itself being rifted apart between Vanuatu and Fiji (Heads 2014). In a discussion of the central Pacific, Holland (2012) criticised the idea that submerged seafloor structures, ‘e.g., Heads’ [2012a] so-called ‘Mid-Pacific Mountains’ ’, were formerly emergent. Holland wrote that ‘this extraordinary idea was proposed in various iterations in the 18th and 19th centuries’ and, even though the idea ‘sounds peculiar today’, the mistake can be attributed to the primitive technology of the time (‘wooden sailing ships, sextants and sounding lines’). Holland suggested that geologists using modern techniques (such as ‘K–Ar radiocarbon dating’ [sic]) have rejected the ‘midPacific plate hypothesis’, and ‘so the panbiogeographers intentionally . . . ignore or misinterpret many important advances in natural science. . .’. Despite Holland’s (2012) claims, no-one has rejected the ‘mid-Pacific plate hypothesis’; it is common knowledge that many Pacific seamounts are former islands; and the term ‘MidPacific Mountains’ is not my own; it is the standard name for a large, well known, submarine mountain range. This range forms a system together with the atolls and seamounts of the Line Islands, and both features are known to include many former high islands (Firth 1993; Natland and Winterer 2005). Geological studies continue to find good evidence for former islands in the region, for example, on the Necker Ridge west of Hawaii (Gardner et al. 2013). These structures have been neglected by the modern dispersalists; however, they are discussed in panbiogeographic work (Heads 2012a). Panbiogeography, its critics, and the case of the ratite birds flawed. . . I discuss here only Hawaii, which provides the type example for rationalization of a ‘plume track’ while disregarding both observed tectonic controls of magmatism and failure of geophysical predictions in plume speculations. . .’. Another study of Hawaii proposed that ‘lithospheric architecture and stress control the locations of volcanoes, not localised thermal anomalies or deep mantle plumes’ (Anderson 2011). Foulger (2012) concluded that the Hawaiian melting anomaly (‘hotspot’) is neither hot nor a spot. According to Holland (2012, p. 317): Heads [2012a] argues further, attempting to cast doubt on established geological principles, that the Pacific hot spot position is not well characterized, that mantle plumes may not exist, that the Hawaiian Islands may have once been joined with other land masses, and ‘In fact the whole subject of intraplate volcanism is currently being debated among geologists’. I wonder which geologists? There is no need to wonder though, because the debate was fully referenced in the book that Holland cited (Heads 2012a). For example, the cited studies include two large volumes that the Geological Society of America has devoted to the topic (Foulger et al. 2005; Foulger and Jurdy 2007). A recent critique of mantle-plume theory has been published by Anderson (2013). Anderson is a leading geophysicist (he is a recipient of the highest scientific honour of the US government, the National Medal of Science), and although the idea that I am the one ‘attempting to cast doubt on established geological principles’ suits Holland’s (2012) story, it is supported only by his deliberate ignorance of the geological literature that I cited. For the Hawaiian Islands, O’Grady et al. (2012) suggested that ‘our well supported understanding of the geologic processes underlying island formation precludes the panbiogeographic persistence of metapopulations in any real sense’. They based this criticism entirely on the idea that in the Hawaiian Islands, ‘there have been times during the past 60 million years where few or no islands existed’. Yet this idea overlooks a serious problem with the calculations it was based on; namely, the method that was used underestimates the heights of present volcanoes, and so it probably also underestimates the heights of past islands (Heads 2012a, p. 319). Because of this, Waters (2007) was justified in suggesting that Hawaiian Drosophila and also some elements of the Galapagos biota are ‘relatively ancient, dating back to historical (now ‘extinct’) islands’. O’Grady et al. (2012) described the suggestion that a metapopulation could persist indefinitely around an island-generating structure as a ‘fantastical conjecture’, but it has been accepted for the Hawaiian Islands by many authors, including dispersalists (Waters 2007) and geologists (Menard 1986), not just panbiogeographers. Holland (2012) wrote that ‘regarding the islands of Maui and Hawaii, and against all geophysical evidence [none was cited], Heads [2012a] states that ‘there may have been land between the two islands’ (p. 357)’. This ignores the excellent geological evidence that I cited for subsidence between the two islands (now 50 km apart) caused by volcanic loading: Australian Systematic Botany 245 Price and Elliott-Fisk (2004, fig. 5) plotted one of the breaks-in-slope (their ‘H terrace’) for 125 km from Molokai, where it is about 500 m deep, to east of Maui, where it is over 2000 m deep. This means that the originally horizontal feature has tilted as it subsided; the tilt has resulted from higher rates of subsidence closer to the current zone of volcanic loading, the volcanoes on Hawaii. The profile of the H terrace, as far as it was mapped, shows direct evidence for at least ~1500 m of subsidence and as the sea between Maui and Hawaii is currently only between 1500 and 2000 m deep there may have been land between the two islands [Heads 2012a, p. 357]. Whether Maui and Hawaii were actually joined or not is unknown and unimportant; the point is that the sea gap between them has increased over time with continued subsidence, and this provides a possible mechanism for vicariance in shallow-water and terrestrial groups. Organisms that could disperse, say, 3 km across open ocean, could have maintained an original metapopulation in Maui and Hawaii, but they may not be able to disperse 50 km, and so, with continuing subsidence, they could have diverged. Chatham Islands biogeography Elsewhere in the Pacific, the Chatham Islands of New Zealand have a distinctive biota that includes many endemics. Panbiogeographic work has argued, in agreement with molecular-clock dates, that at least some of the endemics predate the current islands (Heads 2011). Waters et al. (2013a) wrote that ‘geological and palaeontological analyses . . . clearly show that the islands themselves were completely submerged until less than 10 Ma’ (p. 495). In fact, geologists have agreed that islands existed in the area currently occupied by the Chathams before 10 Ma, but that there were no emergent islands from 6 until 3 Ma (Campbell 2008; Landis et al. 2008; Campbell et al. 2008, 2009). Nevertheless, on the basis of older clock dates for various plants endemic to the Chatham Islands, Heenan et al. (2010) suggested that islands also existed in the region between 6 and 3 Ma. In addition to the islands that geologists have accepted in the Chathams archipelago before 10 Ma, and the islands that Heenan et al. (2010) proposed there between 6 and 3 Ma, there is now excellent evidence for many, now-submerged seamounts (former islands) around the Chatham Islands and the Chatham Rise, lying between the islands and the New Zealand mainland (Rowden et al. 2005). Multibeam surveys of seamounts around part of the Chatham Rise have showed that the largest volcanic cones are ~2000 m in diameter and that most of the cones have flat tops (Collins et al. 2011), indicating erosion to sea level before subsidence. Waters et al. (2013a) argued that: ‘. . .multiple independent lines of evidence clearly indicate that the modern Chatham Islands biota was established by trans-oceanic dispersal [from the mainland]’ (p. 496), but they overlooked the seamounts. Biogeography and orogeny Panbiogeographic work has indicated that present distributions developed on past landscapes, not on present ones. Although the former landscapes no longer exist, they can be inferred from 246 Australian Systematic Botany M. Heads indirect evidence. Waters et al. (2013a) accepted that: ‘. . .the geological record retains little or no preserved evidence of topographic features in 200 million year old orogens’ (p. 496), but, by definition, any orogen, whatever its age, constitutes evidence for topographic features, namely mountains. Waters et al. (2013a) argued that although ‘. . .there remains abundant visible evidence of topographic barriers in young [<5 Ma] mountain belts . . . panbiogeographers apparently ignore the latter events’ (p. 496). Instead, panbiogeographers ‘routinely attribute such biological radiations to ancient (>20 Ma) geological processes (e.g. Heads 1998; Heads and Craw 2004; Heads 2012b, figs 5, 6), but without the quantitative evidence needed to discount younger processes.’ The spatial evidence for rejecting particular features as causative is straightforward. The New Zealand clades mapped in the figures cited (Heads 2012b, fig. 5 showing skinks, and fig. 6 showing harvestmen) have their main distributional breaks at the Alpine fault, not at the main divide formed by the uplift of the young mountains. In addition, the distributions of the two groups each include marked dextral (right-lateral) disjunctions. This means that the breaks cannot be explained by later Neogene uplift without additional, ad hoc hypotheses. Instead, they are consistent with an origin by earlier Neogene strike-slip (horizontal) displacement along the Alpine fault. Despite the claims of Waters et al. (2013a), panbiogeography is not ‘fixated on ancient earth history’ (p. 494), because it also considers the effects of Neogene Earth history. Apart from proposing Neogene strike-slip displacement of communities in New Zealand, panbiogeographic studies have concluded that Neogene orogenic uplift has raised populations in New Zealand, New Guinea, the Andes and elsewhere, and this has been accepted by subsequent authors (for example, Ribas et al. 2007, writing on Andean parrots). Nevertheless, many geographic distributions do not coincide with young mountain belts. Instead, clades in groups such as the New Zealand skinks and harvestmen, cited above, show spatial coincidence with other tectonic features, such as the Alpine fault (Heads and Craw 2004). This can also be seen in ratite birds, as the primary breaks in clades of both kiwis and moas coincide with the Alpine fault, not the main divide. This is discussed in the next section. Ratite birds and plate tectonics The ratites are the sister-group of all other extant birds, and include the largest living birds (ostriches). They also include the elephant birds (Aepyornithidae) of Madagascar, which are largest of all known birds, living and fossil. The Aepyornithidae, along with the moas (Dinornithidae) of New Zealand, went extinct in historical times. All known ratites are flightless, except the tinamous and the fossil group Lithornithidae. Although they were named as a group, Ratitae Merrem, in 1813, the original delimitation of the group has now changed, to include tinamous, and many authors refer to the new grouping as Palaeognathae Pycraft 1900 (Phillips et al., 2010; Waters et al. 2013a). Nevertheless, there is no need to change the name of a group just because its delimitation changes. The name Ratitae, or ratites, is much older and much more widely known than Palaeognathae, and so it is used here. The distribution of the ratites has intrigued biogeographers for many years. Most of the patterns seen in the group, including major trans-oceanic disjunctions, are also well known in other animals and in plants, and so ratites have become a standard test case for biogeographic theory. Waters et al. (2013a) illustrated the methods of panbiogeography using the example of the ratites. The authors showed that the panbiogeographic analysis of the group given in Craw et al. (1999), based on morphology, is inconsistent with aspects of the latest phylogenies (Smith et al. 2013; cf. Mitchell et al. 2014a). This is not surprising, because there has been a molecular revolution in systematics over the past 15 years. The actual distributions of the molecular clades of ratites are more interesting, and, because they have not been mapped before, they are examined here briefly as a case study showing how panbiogeographic methodology works. Molecular phylogeny and distribution of the extant and subfossil ratite clades The extant and subfossil ratites comprise four main clades, with the molecular phylogeny (Mitchell et al. 2014a) and distributions (del Hoyo et al. 1992) shown in Figs 1 and 2. Fossil ratites As indicated above in the section ‘Biogeography and fossils’ section, it is sometimes suggested that panbiogeography ignores the fossil record. This is not correct; any fossils that are known are incorporated into dating and spatial analyses (for example, see Heads 2012a, on primates). The advocates of dispersal theory dispute this, not because panbiogeography ignores fossils, but because it interprets them in a new way. It uses fossils to provide Struthionidae (ostriches): Africa and (fossil) Eurasia Rheidae (rheas): South America south of the Amazon Aepyornithidae (elephant birds) + Apterygidae (kiwis); Casuariidae (cassowaries) + Dromaiidae (emus): trans-Indian Ocean Dinornithidae (moas) + Tinamidae (tinamous): trans-Pacific Ocean Fig. 1. Molecular phylogeny of the extant and subfossil ratites (Mitchell et al. 2014a). Panbiogeography, its critics, and the case of the ratite birds x x x x Australian Systematic Botany x x x x x x Lithornithidae † x 247 x Tinamidae Casuariidae Struthionidae Aepyornithidae † Dromaiidae Dinornithidae 1 Ratites 1 (2 (3 + 4)) † 2 4 3 Apterygidae Rheidae x Fig. 2. Distribution of the ratites (phylogeny from Mitchell et al. 2014a, distributions from del Hoyo et al. 1992). Fossil localities as X symbols. Putative ratite fossils from Seymour Island, northern Antarctic Peninsula, are indicated with an arrow. Two localised fossil groups, Palaeotididae of Germany and Remiornithidae of France, are not shown. The fossil record of Casuariidae from either Wellington Valley (New South Wales) or the Darling Downs (Queensland) is also not shown. minimum ages for clades, not maximum ages, and does not assume that the oldest fossil represents a centre of origin. In practice, many fossils, especially older ones, are very difficult to identify and place on phylogenies, because they do not include many critical morphological features (such as soft parts) or any DNA. In many groups, a great deal of anagenetic evolution, as well as cladogenesis, has occurred over time, and fossils of a lineage from the Cretaceous, say, may be quite different from the modern representatives. In ratites, the affinities of many putative fossil members are controversial. Overall, the fossil record of ratites is very poor. For example, elephant birds have no pre-Pleistocene fossil record. Kiwis (Worthy et al. 2013), moas (Tennyson et al. 2010), ostriches (Mayr 2009) and tinamous (Mayr 2009) have no fossils older than Miocene, and the oldest fossils of the emu–cassowary lineage are Oligocene (Worthy et al. 2014). The oldest rhea fossils are Paleocene (Feduccia 1996). (Note that the sequence of appearance of the families in the fossil record does not follow the phylogenetic sequence.) In addition to the extant clades, ratites include three fossil families. Lithornithidae are known from North America and Europe (Paleocene to Eocene; Fig. 2), but the family’s exact composition, whether or not it is monophyletic, and its position in ratite phylogeny, are all unresolved (Mayr 2005). The affinities of two fossil groups of ratites, the Palaeotididae of Germany and the Remiornithidae of France, are also obscure (Mayr 2009). Their distributions are local and are not shown in Fig. 2. Eleutherornis (Eleutherodactylidae) of Switzerland has been regarded as a ratite, but there is no compelling evidence for this identification (Mayr 2009). Eremopezus (including Stromeria) is represented by Eocene fossils from Egypt and was identified as Aepyornithdae (for example, Feduccia 1996), but it is now placed in its own family, Eremopezidae, and its status as a member of ratites has been questioned (Rasmussen et al. 2001). Some fossil eggshells not associated with bones have been attributed to ratites, but their identification is ambiguous, and ratite-like eggshells are also found in some extant, non-ratite birds (Mayr 2011). Likewise, ‘aepyornithid-type’ eggshells are widespread as fossils in Africa and Asia, but the similarities with Aepyornithidae are likely to represent characters that are plesiomorphic in ratites (Bibi et al. 2006). No fossil ratites are known from mainland Antarctica. There is putative ratite material from Seymour Island off the Antarctic Peninsula, but nothing more can be said about its affinities and even its identification as a ratite needs to be verified (Mayr 2009). Ratites, living and fossil, are widespread globally, and it would not be surprising if fossils were found in India, Sri Lanka, South-east Asia or Antarctica. Feduccia (1996, p. 285) argued that the northern hemisphere fossil ratites ‘almost certainly evolved there long after the breakup of Gondwanaland’, but this assumes, illogically, that fossil dates give maximum rather than minimum clade ages. The northern fossil forms have been described as ‘ancestral ratites’ (Chatterjee 1997, p. 263), but there is no evidence that the fossil families were ancestral to the extant ones. Distribution of the ratite clades All the distribution patterns in the ratites could be the result of chance dispersal events, that is, events that only occurred once in the history of the lineage and were unrelated to any other biological or physical factors. An alternative is a vicariance model, beginning with a widespread ancestor. The main feature of the distribution pattern of ratites (Fig. 2) is its simplicity. Although the group is widespread globally, the four main clades (and the Lithornithidae) are allopatric everywhere except New Zealand and South America south of the Amazon. This has not been mentioned by previous authors, but it is consistent with an origin of the 248 Australian Systematic Botany clades by simple vicariance, followed by dispersal and local overlap within New Zealand and South America. Allopatry as a general pattern has been explained either by founder dispersal or by vicariance. Allopatry by founder dispersal is sometimes accounted for by a ‘founder takes all’ model, with the colonising clade preventing any subsequent invasion by competition (Waters et al. 2013b). In the ratites, this is contradicted by the overlap of clades in New Zealand and South America. A panbiogeographic approach asks the following general question: can a coherent synthesis of a group be constructed in which the phylogenetic and distributional breaks (nodes) in the group are correlated spatially with tectonic events in the same region as the breaks? The phylogenetic and geological breaks must appear in the same chronological sequence, and the clades must be no younger than their oldest fossils or fossil-calibrated clock dates. The first node in the ratite phylogeny separates ostriches (notably absent from Madagascar) from the rest. The break corresponds with the Mozambique Channel, formed at ~160 Ma in the late Jurassic. Other parts of the break occur between China (where there are ostrich fossils) and New Guinea (with cassowaries). The next node separates rheas and Clades 3–5. The extensive overlap of rheas with tinamous obscures the site of the original break; however, whereas rheas do not occur on the western coast of South America, tinamous do, namely along the coast of central Chile at 29–41S (The IUCN Red List of Threatened Species, search ‘Tinamidae’ at www.iucnredlist. org, accessed 23 November 2014). This region has many biogeographic links with Australasia, for example, in marsupials (Heads 2014, pp. 95, 96). Thus, the original break is likely to have been located between rheas in south-eastern South America and its sister in the west, with tinamous subsequently expanding their range eastward. The break between rheas in the east and, for example, tinamous in the west corresponds spatially with a volcanic mega-event, the third, and last pulse of Chon Aike volcanism (138–157 Ma, latest Jurassic–earliest Cretaceous). This occurred along the line of the future Andean cordillera. The Chon Aike volcanic province extends from central Argentina to Tierra del Fuego and West Antarctica, and is one of the largest rhyolitic provinces in the world (Rapela et al. 2005). The volcanism was a precursor of breakup in this part of Gondwana. Trans-Indian Ocean v. trans-Pacific Ocean allopatry The differentiation of the ostriches and rheas leaves a diverse, widespread Indo-Pacific group, comprising Clades 3 and 4, that is examined here in some more detail. It comprises a transIndian Ocean group (Clade 3) and its sister, a trans-Pacific group (Clade 4). A vicariance model for these two clades predicts that other groups will have similar distributions, and examples of these include the following: In the conifer family Podocarpaceae, the trans-Indian Ocean clade Afrocarpus + Nageia is sister to the transPacific Retrophyllum [Heads 2014, fig. 3.20, including living and fossil records]. M. Heads In angiosperms, the trans-Indian Ocean Salvadoraceae is sister to the trans-Pacific pair Bataceae + Koeberlinaceae [Heads 2014, fig. 3.19]. In the butterfly tribe Troidini (including the largest of all butterflies, in New Guinea), the trans-Indian Ocean clade Pharmacophagus + Cressida is sister to the trans-Pacific clade Trigonoptera [Heads 2014, fig. 3.22]. A vicariance history also predicts that the same transIndian v. trans-Pacific pattern will also occur in groups with different ecology, such as shallow-water marine groups. This can be confirmed, because the mangrove genus Rhizophora, for example, comprises a trans-Indian group (R. stylosa and relatives) and its sister, a trans-Pacific clade (R. samoensis and relatives) (Lo et al. 2014). Lo et al. (2014) wrote that their ‘comprehensive biogeographic study’ ‘revealed’ both transPacific and trans-Indian Ocean dispersal events; however, they did not mention the allopatry of the two main clades, or the possibility that the allopatry evolved by simple vicariance. In New Zealand, Cooper et al. (1992) found that kiwis (in the trans-Indian clade) and moas (in the trans-Pacific clade) are not sister groups, and therefore, argued that ‘New Zealand was probably colonized twice by ancestors of ratite birds’ (Cooper et al. 1992). Feduccia (1996, p. 278) described this as an ‘inescapable conclusion’, but this is true only if a particular model of ratite evolution, namely, a centre of origin model, is assumed. As indicated, the allopatric distributions of the known ratites are instead consistent with vicariance of a worldwide ancestor. The ancestral Indo-Pacific clade divided into kiwis and relatives around the Indian Ocean, and moas and relatives around the Pacific, with the boundary running through New Zealand. The Indian and Pacific clades themselves then broke down into their modern subclades, the families. In this model, neither moas nor kiwis colonised New Zealand; they both evolved there. India Ocean–Pacific Ocean allopatry explained by prebreakup magmatism and orogeny In their study of ratites, Phillips et al. (2010) examined the biogeography of the group by estimating clade ages with fossil-calibrated clock dates. They ruled out the presence of the birds in Gondwana and a vicariance history for the group, and instead suggested a dispersal model. Yet this idea depends on the treatment of fossil-based clade ages, namely minimum ages, as maximum ages (Heads 2012b). Mitchell et al. (2014a) concluded instead that ‘molecular dating provides limited power for testing hypotheses about ratite biogeography. . .’, and this is accepted here. Instead of focussing on estimated clade ages, Mitchell et al. (2014a) concentrated on the branching sequence of the phylogeny. As they observed, this is incongruent with the breakup sequence of Gondwana, as seen in the break between the Indian Ocean and Pacific clades. They noted, correctly, that the incongruence eliminates a simple vicariance process caused by Gondwana breakup. But they assumed, incorrectly, that this rules out any vicariance model, and so they accepted that the pattern must have resulted from chance founder dispersal. This argument involves two mistakes, the assumption that the breakup Panbiogeography, its critics, and the case of the ratite birds Australian Systematic Botany process begins (rather than ends) with seafloor spreading, and the idea that the breakup of Pangaea and Gondwana represents the only possible mode of global vicariance. The first step in the differentiation of the modern ratites, namely, the separation of the ostrich lineage from the rest, can be explained by initial Gondwana breakup (including between mainland Africa and Madagascar at the Mozambique Channel), but the breakup of the Indo-Pacific clade cannot. Instead, it is attributed here to tectonic events that took place in the lead up to seafloor spreading. One possible mechanism for the Indian-Pacific break is the development of the Australides. This was a mountain belt located along the margin of southern Gondwana in areas that became southern and western South America, western Antarctica and eastern Australasia (Fig. 3). In New Zealand, the last phases of the Australides uplift are recognised as the Rangitata orogeny. The formation of the Gondwanide belt and its successor, the Australides belt, was caused by subduction and terrane accretion along the Gondwana margin from Neoproterozoic (pre-Cambrian) to late Mesozoic time (Vaughan et al. 2005). The Australides region is well known to biogeographers as a major centre of endemism that does not involve the core cratonic areas of Gondwana including eastern South America, Africa India and Western Australia (Gibbs 2006). Thus Australides (‘Pacific’) distributions are distinct from ‘core Gondwana’ patterns that straddle the Indian and Atlantic Ocean basins. Development of the Australides continued through the Mesozoic until backarc spreading began behind the arc (on the continent side) and led to rifting, backarc basin formation and Gondwana breakup. Whereas the Australides developed as a gently curved feature, convex towards the ocean (Fig. 3), 249 during Gondwana breakup, the orogen was recurved into a semi-circular, concave structure that now surrounds the South Pacific. Along with the subduction, accretion and uplift, the Australides belt was also the site of large-scale magmatism. The Whitsunday volcanic province off eastern Queensland (132–95 Ma) has equivalent rocks in New Zealand (Median batholith; with the last phase, in Fiordland and Nelson, taking place at 130–105 Ma) and in Marie Byrd Land, West Antarctica. This belt as a whole, mainly composed of Early Cretaceous granite and rhyolite, forms the Earth’s largest silicic igneous province (Fig. 4, from Heads 2014). Its emplacement, over ~30 million years, was one of the last events in the history of the Australides. The Australides belt developed over a long period of time and in multiple active phases, with the active zone of subduction, accretion, magmatism and orogeny progressing steadily oceanward. The episodes of tectonism generated a series of large-scale topographic features, including Andeanstyle mountain ranges, foreland and backarc basins, and belts of magmatism. All these would have led to major changes in the vegetation and the fauna, even in birds. It is likely that early ratites could fly (Mitchell et al. 2014a), but despite the fact that they can fly, most birds are ‘extraordinarily sedentary’ (Mayr 1940). (For other, similar quotes from very experienced ornithologists, see Heads 2012a, p. 278). The Whitsunday–Median batholith belt runs through the middle of New Zealand and along eastern Queensland. (In contrast, subsequent rifting took place between New Zealand and Australia). Thus, its location and its timing (Early Cretaceous, following the separation of ostriches and of rheas) together suggest that its development divided the Indo-Pacific Gondwana continental crust Australides Panthalassa oceanic crust Fig. 3. The Australides orogen, developed by subduction of Panthalassa crust beneath Gondwana (movement of Panthalassa crust indicated by arrow; from Vaughan et al. 2005, and Milani and de Wit 2008). 250 Australian Systematic Botany M. Heads New Guinea continental crust Whitsunday Volcanic Province and Median Batholith; active until end of Early Cretaceous Rifting; beginning in Late Cretaceous Queensland and Marion Plateaus W New Zealand Australia E New Zealand 90 Ma East Antarctica West Antarctica Fig. 4. A reconstruction for 90 Ma showing the Early Cretaceous Whitsunday–Median batholith silicic volcanic province and zones of Late Cretaceous rifting (Mortimer et al. 2005). (From Heads 2014, reproduced with permission of Cambridge University Press.) clade of ratites into kiwis and allies around the Indian Ocean, and moas and tinamous around the Pacific basin. Overlap of the two clades has developed subsequently, but only within mainland New Zealand. The break between emus (Dromaiidae) and cassowaries (Casuariidae) Emus and cassowaries, both living and fossil, are allopatric for the great majority of their range, with minor local overlap in north-eastern Queensland (Fig. 2). (A fossil cassowary tibiotarsus, Casuarius lydekkeri, was thought to have been found at Wellington Valley in New South Wales, but its provenance has been questioned; it is perhaps more likely to be from the Darling Downs in Queensland, 200 km west of Brisbane; Worthy et al. 2014). In traditional biogeography, groups in New Guinea have dispersed there from either Asia or Australia. In this model, New Guinea cassowaries have dispersed from Australia, rather than evolving in situ from a widespread, generalised cassowary–emu ancestor. Yet a dispersal model would imply that the New Guinea cassowary species are all nested phylogenetically in the Australian population, and this is unlikely. The overall allopatry between emus and cassowaries is impressive and can be explained by vicariance, with the minor overlap explained by local dispersal. The allopatry around a break in eastern Queensland is a very common pattern; for example, it occurs in marsupials, between Dendrolagus of New Guinea and north-eastern Queensland, and Petrogale, widespread through mainland Australia (Heads 2014, fig. 4.10). In the traditional model, the differentiation between cassowaries and emus is ecological; cassowaries live in tropical rainforests, emus in drier habitats. However, cassowaries live in cold (montane) rainforest, as well as in hot rainforest. There is plenty of cold rainforest in south-eastern Australia, similar to that in montane New Guinea; however, (apart from the dubious record of C. lydekkeri) cassowaries are not recorded there. There are also large areas of dry, open grassland and savanna in New Guinea (especially in the Fly River area), where there are no emus, only cassowaries. Thus, cassowaries are well known from savanna and grassland (Davies 2002), but are absent from this type of vegetation in Australia; emus live in savanna and grassland, but are absent from this type of vegetation in New Guinea. In any case, aridification in Australia began only in the Miocene (Davis et al. 2013), whereas Oligocene fossils from Riversleigh in north-western Queensland) are already recognisable as emus (Dromaiidae) rather than cassowaries (Mayr 2009; Worthy et al. 2014). Thus, simple ecological sorting does not explain the phylogenetic and distributional break between cassowaries and emus. An alternative idea is that emus occupy open habitat because they are in Australia; they are not in Australia because they favour open habitat. Likewise, cassowaries are mainly in rainforest because they are a New Guinea group, and currently New Guinea is mainly covered in rainforest. The original differentiation between emus and cassowaries is attributed here to the final phases of activity in the Whitsunday volcanic province, at ~100 Ma (mid-Cretaceous), followed by rotation of New Guinea away from Australia (Zirakparvar et al. 2013). This implies that the original emu–cassowary clade dispersed north across the Whitsunday-Median batholith belt before the Panbiogeography, its critics, and the case of the ratite birds last active phases along the belt led to the differentiation of the two families. A vicariance model for ratite evolution To summarise, the following sequence can be proposed for the main nodes in the ratite phylogeny and the tectonic events that coincide spatially with them: (1) Break between ostriches in Africa and their Indo-Pacific sister group. Opening of Mozambique Channel at 160 Ma (Late Jurassic). (2) Break between rheas and Clades 3–5. The last pulse of Chon Aike volcanism (138–157 Ma, latest Jurassic–earliest Cretaceous). (3) Break between the Indian Ocean clade (3) and the Pacific clade (4). Rangitata orogeny and earlier phases of emplacement of the Whitsunday–Median batholith igneous province at ~130 Ma (Early Cretaceous). (4) Break within the Indian Ocean clade (3), between the Madagascar-New Zealand group and the Australia–New Guinea group. Seafloor spreading around India plus continued activity in Median batholith, at ~130 Ma (after Node 3, but before separation of Madagascar–India from Antarctica–New Zealand, especially after ~120 Ma) (Early Cretaceous) (Reeves 2014). (5) Break between emus and cassowaries. Last active phases of magmatism in Whitsunday volcanic province at ~100 Ma (mid-Cretaceous). (6) Break within the Pacific clade (4), between the moas of New Zealand and the tinamous of South America. Opening of basins around New Zealand at 84 Ma (Late Cretaceous). This vicariance model of ratite evolution begins with a global ancestor, and so there is no need for intercontinental dispersal in any of the ratite clades themselves, only minor overlap within continents. The overlap of moas and kiwis in New Zealand occurred after their origin, but before strike-slip displacement on the Alpine fault (starting in the Miocene) caused species-level differentiation in each, as discussed below. The vicariance model incorporates all of the available evidence from both the fossil and extant records. Most of the data is from the extant record because the fossil record of ratites is poor. With respect to Lithornithidae, in a vicariance model, the common ancestor of the extant ratites already occupied Europe and North America before lithornithids differentiated as such. The precise mode of origin of the family – its break with its sister-group – is unclear, because the family’s affinities are not known. The panbiogeographic approach, as illustrated here, explains allopatry by vicariance and overlap by dispersal. Extinction is another factor, but happens everywhere, all the time, and so does not account for the neat, mosaic patterns of allopatry that are observed in practice. For example, in the ratites, the global allopatry of the clades is the dominant pattern. The comparatively minor areas of overlap can be explained by dispersal. The absence from India and the Antarctica mainland (living and fossil) is probably the result of extinction. The absence from Australian Systematic Botany 251 New Caledonia and all the Caribbean islands is perhaps more enigmatic (Grenada is just 100 km from Trinidad, where there are tinamous, and these can fly; Cuba is just 200 km from the Yucatán Peninsula, where tinamous also occur). Some further aspects of distribution in New Zealand are discussed next, and illustrate panbiogeographic analysis at a smaller geographical scale. Kiwis (Apterygidae) The trans-Indian Ocean clade of ratites includes a subgroup comprising the kiwis (Apterygidae), endemic to New Zealand, and their sister-group, the extinct elephant birds (Aepyornithidae), only known from Madagascar. Mitchell et al. (2014a) attributed the disjunction to long-distance dispersal, but did not examine whether or not it is a standard pattern. In ducks, a widespread New Zealand clade (Anas chlorotis, A. aucklandica, A. nesiotis and the extinct A. chathamica) is sister to A. bernieri of Madagascar (Mitchell et al. 2014b). Likewise, the beetle family Chaetosomatidae is known only from Madagascar and New Zealand (Leschen et al. 2003). In plants, the New Zealand species Euphorbia glauca (Euphorbiaceae) is sister to E. emirnensis of Madagascar (Riina et al. 2013). (It is possible that unsampled species such as E. borbonensis of Réunion and E. reineckii of Samoa also belong to this group; however, this would have little effect on the overall pattern.) Korthalsella salicornioides (Viscaceae) is in Madagascar, New Zealand and New Caledonia (Aubréville et al. 1967; Molvray 1997). New Caledonia is part of the same continental block as New Zealand (Zealandia), and the biota includes several connections with Madagascar (Heads 2010, supplement). The terrestrial isopod Pyrgoniscus is in Kenya, Tanzania, Madagascar, Lord Howe Island and New Caledonia. (Schmalfuss 2003). Acridocarpus (Malpighiaceae) comprises ~30 species of Africa and Madagascar, and one disjunct species endemic to New Caledonia that is sister to two Madagascan species (Davis et al. 2002). Thus, the same Zealandia–Madagascar pattern occurs in ratites, ducks, crustaceans, beetles and plants, fulfilling one of the preconditions for a vicariance model. The component groups in the pattern all have very different ecology and means of dispersal, suggesting that neither factor is responsible for the pattern. In the model for ratites proposed above, the Madagascar and New Zealand sister-groups differentiated in Early Cretaceous time. The kiwis (Apteryx) comprise two main clades, ‘brown’ and ‘spotted’. Members of the brown kiwi clade show very high levels of genetic structuring, with mitochondrial DNA haplotypes often restricted to local populations (Shepherd et al. 2012). Sequencing studies of extant and subfossil populations showed that the brown kiwi clade comprises two main allopatric groups, with their mutual boundary at the Alpine fault, not at the main watershed ridge of the Southern Alps (Fig. 5; Shepherd et al. 2012; Heads 2014). Baker et al. (1995) suggested that the ancestral brown kiwi population moved north from Fiordland to Haast and then Okarito, where the population diverged, yet this does not 252 Australian Systematic Botany M. Heads explain the cause of the divergence. It could be accounted for if the ancestral brown kiwi population was already widespread through New Zealand before the split. The main phylogenetic break occurs between populations of the southern Apteryx australis clade at the Arawata Valley and Haast Range (near Haast village; Holzapfel et al. 2008), and members of the northern A. mantelli clade at Okarito. The location of the break coincides with the plate boundary, represented by the Alpine fault, and this has not been recognised in prior studies. Most populations of the southern A. australis group lie south-east of the main geographic divide. However, the population at Haast and many in Fiordland lie west of the main divide (and east of the Alpine fault; Fig. 5), and so the main divide does not correspond with the phylogenetic break. Wallis and Trewick (2009) discussed the basal split in the brown kiwi clade and accepted that it was too old to reflect Pleistocene events, such as glaciations. They suggested that it could have been caused by uplift of the Southern Alps, but if the break were due to uplift it would be located at the main divide, not at the fault. In addition to the vertical displacement, large-scale strike-slip (horizontal) displacement has also occurred along the fault, and the observed pattern suggests that this has caused the differentiation. Disjunction A. mantelli clade along the Alpine fault is now documented in more than 200 plants and animals, including kiwis (Heads 1998; M. Heads, pers. obs.). For example, three species in the alpine plant Abrotanella (Asteraceae) each show a major disjunction along the fault (Heads 2012c). Moas (Dinornithidae) The other New Zealand ratites are the moas, giant birds that became extinct in historical times. There are two main clades, namely, the upland genus Megalapteryx, with most records at 900–2000-m altitude, and the remaining five genera, usually found at lower sites (Bunce et al. 2009, fig. 2). Molecular studies of subfossil material have supported the idea that moa populations persisted in many refugia during the Pleistocene glaciations, even in colder, montane areas. For example, Megalapteryx shows high levels of genetic diversity relative to the other genera, and this ‘likely relates to the persistence of upland/montane habitat during both glacial and interglacial periods’ (Bunce et al. 2009, p. 20 650). With respect to the Oligocene marine incursions that flooded parts of New Zealand, Bunce et al. (2009, p. 20 649) agreed that ‘the survival of many endemic vertebrates preserved as early Miocene fossils strongly argues against total submergence’. Both kiwis (Worthy et al. 2013) and moas (Tennyson et al. 2010) are known from early Miocene fossils in central Otago. Megalapteryx shows deep phylogenetic divisions with four distinct clades, all in the South Island (Fig. 6; Bunce et al. 2009). The clades are allopatric, with one each in Fiordland, Otago, Nelson and Marlborough–northern Canterbury. The Fiordland clade is sister to the clade in Nelson, and so the pattern is compatible with Alpine fault displacement. Bunce et al. (2009) thought the break between the Fiordland and Nelson Megalapteryx was caused by Pleistocene glaciations, but Megalapteryx favoured colder, upland habitat, and so, during colder phases, its range would probably have expanded † † Megalapteryx † 1 (2 (3 + 4)) † 4. Nelson Okarito † Haast Ra. main divide Alpine fault Fiordland 2. Marlborough Alpine fault main divide A. australis clade † Apteryx clade (brown kiwis) Fig. 5. The two clades of brown kiwi: Apteryx australis species group (light grey) south-east of the Alpine fault, and A. mantelli species group (dark grey) north-west of the fault. Dagger symbols indicate sequenced, subfossil samples from populations now extinct (Shepherd et al. 2012). (From Heads 2014, reproduced with permission of Cambridge University Press.) x x x SOUTH ISLAND 3. Fiordland 1. Otago Fig. 6. The four clades of the moa genus Megalapteryx (Bunce et al. 2009). Localities with unsequenced fossils as X symbols (Worthy 1997). Panbiogeography, its critics, and the case of the ratite birds rather than contracted. Bunce et al. (2009, supporting information, p. 2) acknowledged that ‘the phylogeographic distribution of the Megalapteryx clades does not match their phylogenetic relationships’. This is because the Fiordland clade is sister to the Nelson clade (disjunct along the Alpine fault), not the adjacent Otago clade. The pattern is consistent with the Fiordland + Nelson population and the Otago population breaking apart before the Alpine fault split the Fiordland + Nelson clade. It might be objected that the recent divergence date between Megalapteryx and the other moas calculated by Bunce et al. (2009), 5.8 Ma, rules out Alpine fault disjunction. Nevertheless, this date is fossil-calibrated and so represents a minimum clade age. For example, the calibrations employed by Bunce et al. (2009, supporting information) include: ‘Casuariidae (emu v. cassowary). Normal distribution, median at 25.5 Ma [the oldest emu fossil date], 95% range from 16 to 35 Ma.’ In other words, the maximum possible age of emu and cassowary is stipulated as a ‘prior’, before any analysis, as 35 Ma (with 95% credibility), no more than 9.5 million years older than the oldest fossil in the group. But why 35 Ma? Why not 36 or 37, or 47, or 67 Ma. . .? The date is arbitrary. (For more details on Bayesian dating, see Heads 2012b.) Thus, although the topology of the Bunce et al. (2009) phylogeny is of great interest, the fossil-calibrated dates cannot be accepted as maximum clade ages. A casual observation might suggest that the distributions of the Megalapteryx clades reflect current climate, with the western clades occurring in the wet forests to the west of the Southern Alps and the eastern clades found in drier areas east of the mountains. However, the pattern is not that simple. For example, the eastern Clade A is recorded in areas (south of the Nelson lakes, for example) that are wetter than substantial areas occupied by the western clade B + C (north of the Nelson lakes, for example). The sampling in the study by Bunce et al. (2009) could be improved, but the Megalapteryx clades already appear to be a typical case of Alpine fault disjunction, as in brown kiwis (see above). Conclusions To summarise, none of the criticisms of panbiogeography can be substantiated. It is not creationist, it does not ‘reject dispersal’, its analyses are not ‘disingenuous’, and it does not ignore or misrepresent key evidence. 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