Climatic ameliorations are one of the major abiotic driving forces of cladogenesis in plants and animals (Goldblatt & Manning, 2002; Linder, 2003; Daniels et al., 2006; Gouws et al., 2010; Schnitzler et al., 2011). Historically, these climatic oscillations have resulted in significant habitat fragmentation causing temporal and spatial shifts in species distribution patterns, promoting isolation and speciation (Hewitt, 2003). Whereas the impact of historical climatic changes is well documented and understood in the northern hemisphere, limited comparative studies have been conducted in the southern hemisphere. Forest biome biogeography appears particularly neglected, and evolutionary affinities among them remain poorly understood in southern Africa. In southern Africa and more specifically the south-western Cape (or Cape Floristic Region), the early Miocene was characterized by the presence of subtropical rainforest (Coetzee, 1993). A dramatic climate shift from warm, tropical conditions to drier, more seasonal conditions occurred during the Pliocene and prompted the development of drought-resistant vegetation (Linder, 2003; Cowling et al., 2009). This climatic shift was attributed to the development of the Benguela upwelling system along the west coast of southern Africa during late Miocene (Siesser, 1980). Furthermore, this was a period when the Cape Floristic Region (CFR) underwent a phase of geomorphic evolution with tectonic uplift (Partridge & Maud, 2000). Dramatic geotectonic uplift (600–900 m) in eastern southern Africa increased the east–west rainfall gradient, reinforcing western CFR aridity by intercepting a greater proportion of rainfall from the warm Agulhas current (Tyson & Partridge, 2000). This effectively divided the CFR into a western ‘winter rainfall zone’ (WRZ) and an eastern ‘year-round rainfall zone’ (YRZ). In the western CFR, seasonal drought provided an environment conducive to regular lightning-induced fires. The CFR fire regime is suggested to have become established < 6–8 million years ago, during the late Miocene (Bytebier et al., 2011). This promoted the evolution of the pyrophytic Cape fynbos vegetation (Mucina & Rutherford, 2006; Cowling et al., 2009; Swart et al., 2009). Aridification became increasingly more marked during the Pliocene/Pleistocene epoch with moderate marine transgression and regression. These developments resulted in considerable habitat fragmentation among forested areas, restricting forest habitats to high-lying areas.
Within the CFR, the Afromontane forests represent the smallest biome and are comprised of two subtypes, namely southern Afrotemperate and southern coastal forest (Castley & Kerley, 1996; Mucina & Rutherford, 2006). These forest patches generally occur below 1000 m, in areas where rainfall exceeds 600 mm (Rutherford & Westfall, 1986; Mucina & Rutherford, 2006). The impacts of historical climatic amelioration on forested areas remain largely unexplored in the absence of phylogeographic studies on forest-dwelling taxa. However, it would be reasonable to assume that the increased aridification experienced during the Miocene/Pliocene has fragmented forest habitats. It has been demonstrated that habitat specialists such as soft-bodied invertebrates (e.g. land planarians and springtails) can be effectively employed to reconstruct the biogeographic patterning of forested areas (Garrick et al., 2007; Carnaval et al., 2009; Álvarez-Presas et al., 2011). Their dependence on stable microenvironments and highly restricted dispersal ability make them suitable for reconstructing biome affinities (Garrick et al., 2007; Álvarez-Presas et al., 2011). Onychophora, commonly known as velvet worms, are habitat specialists, restricted to moist environments like closed-canopy forests where they typically inhabit saproxylic environments (Hamer et al., 1997; Daniels & Ruhberg, 2010). Their habitat specificity and general physiological intolerance of desiccation coupled with their low dispersal capabilities render these organisms ideal to test the contractions and expansions of their habitats (Hamer et al., 1997; Álvarez-Presas et al., 2011). The Cape velvet worm, Peripatopsis capensis Grube, 1866, has a relatively wide distribution in the Western Cape of South Africa where conspecific populations are confined to discontinuous, Afromontane forest areas and adjacent fynbos along the Cape Fold Mountains (Brinck, 1957; Hamer et al., 1997). This taxon provides the ideal template organism with which to explore the impact of climate and topography on Afromontane forest in the Western Cape. Daniels et al. (2009) demonstrated that P. capensis comprises three distinct clades corresponding to three distinct biogeographical regions in the Western Cape. The latter study further suggested a historical link between the south-western Cape and the Cape Peninsula forests based on the close phylogenetic relationships of the respective sampling localities; however, limited geographical coverage of the species distribution precluded biogeographic inferences.
In the present study, P. capensis was extensively sampled from Afromontane forests throughout the Western Cape Province of South Africa. The following three hypotheses are explored: (i) due to its habitat specificity, P. capensis will display a genetic history that mirrors the palaeogeography of CFR forests in the light of climatic and geological perturbations, (ii) populations of P. capensis are isolated due to the inhospitality of low-lying coastal plains and the absence of low-lying forests and (iii) increased levels of genetic differentiation should be observed along a west-to-south-easterly trajectory because the south-eastern parts of the Cape Fold Mountain chain harbour larger fragments of forest patches and more pronounced habitat heterogeneity and have historically received higher levels of rainfall.