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).