Between-species divergence and within-species conservatism in climate niche space drive the distinct patterns of genetic variation in two flying squirrel species of Taiwan

Geographic isolation and environmental suitability affect genetic differentiation. Both niche conservatism and niche divergence contribute to differentiation: conservatism may facilitate allopatric speciation, and divergence may facilitate isolation by adaptation. Yet, empirical investigation of the relative importance of these phenomena is complicated by various factors, including differences in topography and geographical features. In this study, we explore the impact of elevation on genetic diversification by contrasting a closely related pair of species, the giant flying squirrels Petaurista lena and Petaurista grandis in Taiwan, with similar life histories but different altitudinal ranges. Both species exhibit a north-south genetic differentiation, with little association between genetic variation and environmental characteristics after accounting for neutral factors, indicating the lack of evidence of niche divergence (isolation by adaptation) as the cause of within-species genetic structure. Instead, historical ranges assuming species-wide niche conservatism corresponds well to the demography. During era with lower temperature, P. lena did not exhibit strong distribution range difference, aligning with the stable population size after the north-south divergence. For P. grandis, on the other hand, the northern population experienced a strong bottleneck coinciding with the drastically reduced suitable ranges in northern Taiwan, likely resulting in the slightly stronger north-south divergence in P. grandis than P. lena despite the former's relatively recent north-south divergence. Therefore, the between-species divergence and within-species conservatism in climatic niche space together led to the distinct past and present range sizes of the two species, resulting in their contrasting patterns of north-south divergence and demographic histories.