Assessing the tempo and mode of avian speciation, and the distribution and associations of avian parasites at inter- and intra-continental scales
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Disentangling historical patterns, processes and events that have resulted in present day numbers and distributions of species has long been a challenge to biogeographers. For decades, biogeographers have sought to uncover simple patterns such that species distributions and area relationships can be fit to a single evolutionary model. These have generally been based on a null hypothesis of vicariance (e.g., Cracraft 1988, Cracraft and Prum 1988, Zink 1996, Riddle et al. 2000). Other studies have shown that while vicariance is clearly important, long-distance dispersal, extinction, and speciation across ecotones all play roles in explaining distributions (e.g., Joseph and Moritz 1993, Smith et al. 1997, Juste B. et al. 1999, Voelker 1999, 2002a, b, McLennan and Brooks 2002, Ogden and Thorpe 2002, Raxworthy et al. 2002, Bowie et al. 2004, Fuller et al. 2005, Outlaw et al. 2007, Voelker et al. 2009, 2014). It is increasingly evident that single evolutionary patterns are insufficient to explain historical biogeographic and biodiversity patterns (e.g., Voelker et al. 2013). This is evident when diverse lineages and areas subject to potentially diverse interactions (as most are) are considered.Perhaps nowhere is our knowledge of avian speciation mechanisms and biogeographic pattern more limited than it is within Africa and Asia (excluding Europe). Despite the fact that open dry habitats prevail in Africa, most molecular-based studies of speciation and biogeography have been focused on tropical forest and tropical montane habitats, and refugial theories (e.g., Fjeldså 1994, Fjeldså and Lovett 1997, Roy 1997, Beresford and Cracraft 1999, Bowie et al. 2004, 2006). Even within these Afrotropical forests, it is clear that our understanding of species-limits is poor, such that extensive cryptic diversity and by extension more intricate patterns of diversification are evident (e.g., Huntley and Voelker 2016, 2017, Voelker et al. 2017, Huntley et al. 2018 and in press). By studying a broader suite of avian genera than has been accomplished to date, large scale patterns of continent to continent, continent to island, and within continent movements and speciation may become far clearer than at present. For example, species relationships (along with molecular clock data) can provide additional insights into the relative roles that the trans-Mediterranean and Indian Ocean Island colonization routes might have played in Asian-African interchange (Voelker and Outlaw 2008). While studies have implicated the former route only in recent colonizations (< 6,000 years before present; Voelker 1999, 2002a, b), it is possible that different genera may show a wider temporal range of historical usage of this route. Prior to 6,000 years ago, a cooling trend was in effect over northern Africa (Kutzbach and Liu 1997) such that grassland corridors existed across the Saharan Desert, and a pre-Pleistocene land bridge existed across the Mediterranean (Moreau 1952). Grasslands across the Sahara and a Mediterranean land bridge may have facilitated movement between Europe and Africa..........