Introduction
Montane ecosystems are highly valuable to the study of evolutionary and genetic consequences of environmental heterogeneity due to the rapid shifts in environmental variables (e.g. temperature, precipitation, and solar radiation) over short distances. Relative to those in the temperate zone, tropical mountains experience less seasonal temperature variation which generates greater elevational stratification (Polato et al., 2018). Janzen (1967) hypothesized that this temporal thermal stability paired with spatial environmental heterogeneity should select for narrow thermal tolerances, which in turn result in low effective dispersal and population isolation across elevational gradients (Ghalambor, Huey, Martin, Tewksbury, & Wang, 2006; Gill et al., 2016). Consistent with Janzen’s (1967) hypothesis, many studies have documented narrower elevational ranges among tropical montane species than temperate species across diverse taxonomic groups (McCain, 2009; Ghalambor et al., 2006). Fewer studies have tested the prediction stemming from Janzen’s (1967) hypothesis that restricted gene flow among populations spanning elevational gradients results in genetic divergence. The available data regarding this prediction are contradictory - some studies have found significant population genetic divergence across elevations, for example, in insects (Polato et al., 2018; Gueuning et al., 2017) and birds (Gadek et al., 2018; DuBay & Witt, 2014; Linck, Freeman, & Dumbacher, 2019). Others have found high rates of gene flow, sometimes in the presence of adaptive phenotypic divergence (Cheviron & Brumfield, 2009; Gadek et al., 2018; Branch, Jahner, Kozlovsky, Parchman, & Pravosudov, 2017).
Very few studies have investigated the population genetic structure of small mammals across elevational gradients in tropical montane ecosystems (Muenchow, Dieker, Kluge, Kessler & von Wehrden, 2018; but see Yu, 1995). We address this knowledge gap by elucidating the population genetic structure of the mountain treeshrew, Tupaia montana , across its full elevational range on two mountains in Kinabalu National Park (KNP), Sabah, Borneo: Mt. Kinabalu and Mt. Tambuyukon (Figure 1). The mountain treeshrew provides an interesting system in which to study the effect of environmental gradients on population structure because it has a broad elevational distribution compared to other small mammals in KNP (Camacho-Sanchez, Hawkins, Tuh Yit Yu, Maldonado, & Leonard, 2019; Nor, 2001).
Understanding the population genetic structure of tropical montane taxa like the mountain treeshrew is important for conservation because it enables researchers to identify metapopulation dynamics and distinct evolutionary units warranting protection and to predict and track species’ responses to changing environmental conditions (Moritz, 1994; Castillo Vardaro, Epps, Frable, & Ray, 2018; Camacho-Sanchez et al. 2018). This is critical given the vulnerability of tropical montane ecosystems to the impacts of global climate change (GCC) (Feeley, Stroud, & Perez, 2017; Lenoir & Svenning, 2015) and the paucity of population genetic studies in Southeast Asia (Muenchow et al., 2018).