Karst habitat islands drove a largely nonadaptive and allopatric radiation in Petrocodon (Gesneriaceae)

Whether evolutionary radiations are primarily driven by ecological or neutral processes remains an unresolved question. The karst-endemic genus Petrocodon, distributed across southern China and northern Indochina, provides a powerful system for testing these alternatives. We generated whole-genome sequencing data for 54 of the 60 described species and integrated these with morphological, geographic, and paleoclimatic data to evaluate the relative roles of ecological and neutral processes in diversification. Our analyses revealed a robust nuclear phylogeny but extensive cytonuclear discordance, likely due to pervasive ancient hybridization and introgression. Macroevolutionary and biogeographic analyses indicate that diversification began during a favorable climatic period enabling range expansion, but subsequent species accumulation was primarily driven by geographic isolation and allopatric divergence rather than ecological niche differentiation. Morphological evolution decoupled from lineage diversification, with most morphological diversity accumulating relatively late. Further analyses support genetic drift in small, isolated populations driving morphological evolution, although stabilizing selection cannot be excluded, and reticulate evolution may have additionally contributed to phenotypic innovation. Together, these results support a largely nonadaptive radiation in Petrocodon, in which spatial opportunity played a central role in species accumulation, while demographic and reticulate processes may have contributed to its striking phenotypic diversity.