4.3 Integrative species diversification studies
Prior to the availability of genomic data, dated phylogenies,
paleo-distribution modeling, and statistical analyses of historical
demography (Carstens & Richards, 2007; Ellegren, 2014; Knowles, 2009;
Knowles & Madison, 2002; Luikart, England, Tallmon, Jordan, &
Taberlet, 2003), biogeographers commonly observed patterns of species
distributions from which they attempted to infer mechanisms of
diversification (e.g. Avise, 2000; Templeton, 2001). The majority of
rainforest refugial studies were conducted in this descriptive,
pattern-based manner, usually employing only single-locus mitochondrial
DNA data sets (e.g. Fjeldså & Lovett, 1997; Haffer, 1969; Mayr &
O’Hara, 1986). The pitfalls and lack of power in such approaches have
been discussed elsewhere (e.g., Knowles & Madison, 2002; Provan &
Bennett, 2008; Stewart, Lister, Barnes, & Dalén, 2010), and are not the
focus of this study. However, with respect to hypotheses of tropical
African species diversification, it is noteworthy that so many of the
concepts and hypotheses surrounding refugia have only been evaluated by
single-locus data sets during the era of mitochondrial gene
phylogeography (but see more recent, multilocus examples in Leaché et
al., 2019; Portik et al., 2017).
The complexity of geographic barriers in West and Central Africa, and
the association of refugia with areas of high surface relief or riparian
zones (Hofer et al., 1999; 2000; Fig. 1), makes it extremely difficult
to untangle the relative importance of different diversification
mechanisms with distribution data alone (Leaché et al., 2019; Portik et
al., 2017). This difficulty is particularly salient in our study system,
where distribution data may have suggested the association of
populations with hypothesized refugia around the Congo River, Gabon, and
in West Africa (refugia 9, 5–8, and 1–3 respectively, Figs. 1, 2).
Yet, our dated phylogenies and paleo-distribution models reject the
Pleistocene population age and habitat contraction predictions of the
refugial hypotheses in favor of the river barrier hypothesis. These
results highlight the importance of using an integrative,
multidisciplinary approach to statistically distinguish among competing
hypotheses to explain high levels of geographically concentrated species
biodiversity. Moving beyond pure pattern-based inference, a deeper and
more nuanced understanding of the production, partitioning, and
maintenance of diversity in complex landscapes may lead to inference of
environmental and evolutionary processes that accumulate terrestrial
biodiversity in tropical areas, which coincide in many cases with Global
Biodiversity Conservation Hotspots (Hrdina, & Romportl, 2017;
Mittermeier, Myers, & Mittermeier, 2000; Mittermeier, Turner, Larsen,
Brooks, & Gascon, 2011; Myers, 1988) and other imperiled ecosystems of
Earth.