Allopatric speciation and interspecific gene flow driven by niche conservatism of Diploderma tree lizards in Taiwan

Allopatric speciation is a widely accepted hypothesis for species
distributed across geographic barriers. Meanwhile, niche conservatism, the
tendency of species to retain their ancestral ecological traits, helps
reinforce genetic differentiation by stabilizing species distributions
over time and reducing the role of competition in shaping range
boundaries. In contrast, hybridization can occur at the edges of
distribution after secondary contact following climatic or geological
events, leading to a reduction in genetic divergence between divergent
lineages. In this study, we investigated the role of geographic barriers,
niche conservatism, and gene flow in the speciation history of Diploderma
species in Taiwan, where geographically distinct taxa share similar
environmental preferences. By using ddRAD-seq data, seven distinct genetic
clusters were identified with two putatively new cryptic species in D.
brevipes and D. polygonatum. Most sister species pairs share similar
climatic niches based on niche equivalency and similarity tests. We
further detected significant historical gene flow between lineages of D.
brevipes and D. polygonatum, where secondary contact might have occurred
because of paleo-climate changes and historical demographic expansion. Our
results demonstrate that niche conservatism does not always act in concert
to strengthen the result of allopatric speciation; instead, it may also
lead to gene flow between divergent lineages following secondary contact.
On the other hand, post-divergence gene flow may be a creating force
generating phenotypic diversity in sexually selected traits in our study
system. The underestimated species diversity of Diploderma in Taiwan
requires further taxonomic works in the future.