The Centre-Periphery Hypothesis(CPH) traditionally suggests that the species' core populations, thriving in optimal environments, exhibit high diversity and resilience, while edge populations display reduced genetic diversity and pronounced genetic differentiation. However, global climate change, complex population dynamics, and diverse landscapes can disrupt these expected patterns. Investigating the drivers of these deviations is critical to understanding the mechanisms shaping species' genetic patterns.

Researchers from the Wuhan Botanical Garden of the Chinese Academy of Sciences analyzed the effects of historical demographic processes and environmental factors on spatial patterns of genetic variation of two iconic tree species, Liquidambar formosana and L. acalycina, a sister pair of East Asia's Tertiary relict forests.

Results revealed that although both species diverged into two lineages within their respective populations (L. formosana: ~2.7 Ma; L. acalycina: ~2.5 Ma), their post-divergence histories differed markedly. L. formosana experienced cyclical population contractions and expansions, while L. acalycina underwent continuous population decline.

"Our findings highlight the complexity of genetic diversity patterns in these relict species,” explains Professor QIU Yingxiong, at the Wuhan Botanical Garden. "L. acalycina largely conforms to the CPH, with core populations exhibiting higher genetic diversity, while, L. formosana represents a different story.”

L. acalycina edge populations, particularly those in high-altitude western and southwestern regions showed heightened genetic differentiation and elevated genomic vulnerability due to long-term geographic isolation and limited adaptive genetic loci. In contrast, L. formosana displayed no clear core-to-edge genetic diversity gradient, despite having the highest population density in its core range. This anomaly was linked to rapid population expansions during the Early Pleistocene, which reshaped its genetic structure. Further analysis showed that L. formosana retained abundant adaptive loci associated with drought and heat tolerance, likely facilitated by heterogeneous landscapes. This resulted in lower genomic vulnerability and stronger adaptability campared to L. acalycina.

The contrasting evolutionary trajectories of these two species underscore the importance of considering historical demographic processes, geographic isolation, and climatic shifts collectively, when assessing genetic diversity. These findings provide critical insights for conserving Tertiary relict tree species under ongoing climate change. 

The research was supported by the National Natural Science Foundation of China International Cooperation Program, the National Key Research and Development Program of China, and the Zhejiang Provincial Key Agricultural Science and Technology Project. It was published in Journal of Biogeography, titled“Evaluating the Centre-Periphery Hypothesis Through Genomic Phylogeographical Comparisons of Two Sister Species of Liquidambar in East Asia's Tertiary Relict Forests".

Population demographic history of Liquidambar formosana and L. acalycina (Image by WBG)

Patterns of genetic diversity distribution in Liquidambar formosana and L. acalycina (Image by WBG)