update: Divergence of island biotas when they were not always islands

Abstract

ISSN 1948‐6596 news and update update Divergence of island biotas when they were not always islands Islands have frequently captured the attention of biogeographers, providing insights into oceanic dispersal capabilities (McDowall 2002), the proc‐ esses of adaptation and diversification (Darwin 1872, Losos and Ricklefs 2009), and immigration‐ extinction dynamics (MacArthur and Wilson 1967). Furthermore, the presence of related but distinct lineages on islands and continents is con‐ sidered to represent classic examples of allopatric speciation (Mayr 1942). However, for continental shelf islands—those that possessed terrestrial connections to other landmasses via exposed con‐ tinental shelf during low sea stands (Whittaker and Fernández‐Palacios 2007)—the interpretation may not always be so simple. Continental shelf islands are common throughout the globe, ranging from the numerous small nearshore islands that dot our coastlines, to larger and more notable examples, represented by places such as Tasmania, New Guinea, Borneo, Sri Lanka, Taiwan, Japan, England and the Falkland Islands (Malvinas). Continental shelf islands, like oceanic islands, often house a variety of endemic species (Kallimanis et al. 2010), and divergence resulting from the development of island‐ mainland allopatric distributions is typically in‐ voked (Fig. 1a). However, continental shelf islands have repeatedly and predominantly experienced terrestrial connections to mainlands throughout the Pleistocene, represented by glacial low sea stands (Lambeck et al. 2002). Therefore, the diver‐ gence of continental shelf island biotas from those on the mainland may have occurred by various means (allopatric, peripatric, parapatric or sym‐ patric speciation; Coyne and Orr 2004) prior to the development of island‐mainland allopatric distri‐ butions themselves (e.g. Fig. 1b,c). A recent study from Taiwan illustrates a novel approach by which the divergence of sister lineages can be tested for deviation from a strictly allopatric island‐mainland model. Li et al. (2010a) studied Taiwanese and Chinese hwamei (Leucodioptron taewanus and L. canorum canorum, respectively), representing species of babbler that are sedentary and occupy the mar‐ gins of secondary broadleaf evergreen woodlands. Figure 1. Hypothetical divergence scenarios leading to contemporary island‐mainland sister‐lineages. Cir‐ cles of different colours indicate the geographic distribution of diverging lineages. The grey shaded areas indi‐ cate continental shelf exposed dur‐ ing Pleistocene glaciations between China and the continental shelf is‐ land of Taiwan. (a) Strict island‐ mainland allopatric divergence. Is‐ land and mainland populations of a widespread ancestor were isolated following a marine transgression. (b) Divergence under gene flow. Diver‐ gence of lineages occurred despite sympatry or parapatry and gene flow, with island‐mainland allopatric distributions formed subsequently. (c) Divergence of lineages within a region (allopatric within the mainland, in this case), with island‐ mainland allopatry formed subse‐ quently. frontiers of biogeography 3.4, 2012 — © 2012 the authors; journal compilation © 2012 The International Biogeography Society