Regional concordance

One of the earliest and best-known examples in which molecular data have identified concordant regional phylogeographical patterns in multiple species was reported by Avise in 1992 (Avise, 1992). He compared the genetic distributions of 19 freshwater, coastal and marine species (15 fish, one bird, one reptile and one mollusk) inhabiting southeastern USA. Twelve of these species, representing both freshwater and marine taxa, showed comparable genetic divisions between the Gulf of Mexico and the Atlantic populations, suggesting that vicariance has similarly influenced the population histories of multiple taxa in this region. Marine taxa are divided by the Florida peninsula, which extends into subtropical waters thereby effecting a barrier to species that live in the temperate waters along the Atlantic coast and the Gulf of Mexico. At the same time, many freshwater taxa are divided by the eastern and western river drainage systems that enter the south Atlantic and the Gulf, respectively. This study has become embedded in the literature as a classical example of comparative phylogeography and is nicely reviewed by Avise (2000).

Since then, as molecular phylogeographic studies have accumulated, other regional examples of concordance have emerged from various geographical regions around the world. In the wet tropical rainforests of northeastern Australia, mitochondrial DNA sequences from several bird, reptile and frog species reflect the division of species into southern and northern clades. The genetic break, which occurs in an area known as the Black Mountain Corridor, must represent a fairly ancient split because conspecific populations from either side of this corridor have sequences that diverge by up to 14.4 per cent (Joseph, Moritz and Hugall, 1995; Schneider, Cunningham and Moritz, 1998). The concordance in this region suggests vicariant population differentiation either side of the corridor, most likely a result of the rainforest repeatedly contracting into separate refugia north and south of the Black Mountain Corridor during the climatic oscillations of the Quaternary period.

Considerable interest has been directed towards the possible role that vicariant mechanisms may have played in creating the extraordinarily high levels of biodiversity in the lowland forests of the Amazon Basin. If vicariance has been particularly common in this part of the world, populations could have regularly become isolated from one another and this in turn could lead to a relatively high speciation rate. One possible mechanism for vicariant speciation in the Amazon Basin is outlined in the riverine barrier hypothesis that dates back to Alfred Russel Wallace, a contemporary of Darwin who made invaluable contributions to the development of evolutionary theory. Wallace spent a number of years in South America, during which time he observed that rivers often seem to create boundaries to species communities (Wallace, 1876). Many years later, the riverine barrier hypothesis was supported by molecular studies that provided evidence for very low gene flow between the populations of some forest understorey birds and saddle-back tamarins (Saguinus fuscicollis) either side of the Rio Jurua, a major tributary of the Amazon River in Brazil (Capparella, 1992; Peres, Patton and Da Silva, 1996).

In contrast, studies of frogs and small mammals have provided little support for the riverine barrier hypothesis (Da Silva and Patton, 1998; Gascon et al., 2000). In these species, the phylogeographic divisions between populations tend to occur not on either side of the river but, instead, between upstream and downstream populations. These phylogeographic boundaries in the central section of the river are longstanding; conspecific populations from the two areas have sequences that diverge by as much as 13 per cent (Da Silva and Patton, 1998; Lougheed et al., 1999). The age and pervasiveness of this division have led to an alternative hypothesis for Amazonian vicariance known as the ridge hypothesis. This is based on evidence for major ridges, or arches, that originated 5-10 million years ago when the Andes were forming, and that define a series of basins in western lowland Amazonia. These ridges have been obscured over the years by an accumulation of sediment but one of them, the Iquitos ridge, is an underlying geological structure that runs perpendicular to the Rio Jurua. In several species, the break between major phylogeographic groups occurs around the site of the Iquitos ridge.

The dart-poison frog (Epipedobates femoralis; Figure 5.13) is one species that supports the ridge hypothesis. Populations sampled from either bank of the Rio Jurua were not monophyletic and had a maximum sequence divergence of 6.13 per

Figure 5.13 A dart-poison frog (Epipedobates femoralis). Photograph provided by Claude Gascon and reproduced with permission

cent. In contrast, populations from upstream and downstream, i.e. from either side of the Iquitos Arch, did form monophyletic groups and, at up to 12.36 per cent divergence, showed substantial differentiation from one another (Lougheed et al., 1999; Figure 5.14). As data from a growing number of species become available, we must conclude that population differentiation and biodiversity around the Rio Jurua have been facilitated by both the river itself and the Iquitos Arch. This example illustrates how phylogeographic patterns in any particular region will, in all likelihood, have been influenced by a number of different factors. In the next

Figure 5.14 Representation of the Rio Jurua in the Amazon Basin, showing the orientation of the Iquitos Arch. Numbers represent the dart-poison frog populations that were sampled by Lougheed etal. (1999). If the riverine barrier hypothesis was correct, we would expect higher genetic similarity among populations on the same side of the river compared with those on opposite sides (populations 2, 3, 5 and 7 versus populations 1, 4 and 6). In fact, the greatest genetic differentiation was seen between populations on either side of the Iquitos Arch (populations 1 and 2 versus populations 3-7), thereby lending support to the ridge hypothesis. Adapted from Lougheed et al. (1999)

Figure 5.14 Representation of the Rio Jurua in the Amazon Basin, showing the orientation of the Iquitos Arch. Numbers represent the dart-poison frog populations that were sampled by Lougheed etal. (1999). If the riverine barrier hypothesis was correct, we would expect higher genetic similarity among populations on the same side of the river compared with those on opposite sides (populations 2, 3, 5 and 7 versus populations 1, 4 and 6). In fact, the greatest genetic differentiation was seen between populations on either side of the Iquitos Arch (populations 1 and 2 versus populations 3-7), thereby lending support to the ridge hypothesis. Adapted from Lougheed et al. (1999)

section we will see examples of the even greater complexities that surround comparative phylogeography at a continental scale.

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