Phylogenetic diversity of two geographically overlapping lichens: isolation by distance, environment, or fragmentation?

Aim Phylogenetic diversification is a precursor to speciation, but the underlying patterns and processes are not well‐studied in lichens. Here we investigate what factors drive diversification in two tropical, morphologically similar macrolichens that occupy a similar range but differ in altitudinal and habitat preferences, testing for isolation by distance (IBD), environment (IBE), and fragmentation (IBF). Location Neotropics, Hawaii, Macaronesia. Taxon Sticta andina, S. scabrosa (Peltigeraceae). Methods We analysed 395 specimens from 135 localities, using the fungal ITS barcoding marker to assess phylogenetic diversification, through maximum likelihood tree reconstruction, TCS haplotype networks, and Tajima's D. Mantel tests were employed to detect structure in genetic vs. geographic, environmental, and fragmentation distances. Habitat preferences were quantitatively assessed by statistical analysis of locality‐based BIOclim variables. Results Sticta andina exhibited high phenotypic variation and reticulate phylogenetic diversity across its range, whereas the phenotypically uniform S. scabrosa contained two main haplotypes, one unique to Hawaii. Sticta andina is restricted to well‐preserved andine forests and paramos, naturally fragmented habitats due to disruptive topology, whereas S. scabrosa thrives in lowland to lower montane zones in exposed or disturbed microsites, representing a continuous habitat. Sticta scabrosa showed IBD only across its full range (separating the Hawaiian population) but not within continental Central and South America, there exhibiting a negative Tajima's D. Sticta andina did not exhibit IBD but IBE at continental level and IBF in the northern Andes. Main conclusions Autecology, particularly preference for either low or high altitudes, indirectly drives phylogenetic diversification. Low diversification in the low altitude species, S. scabrosa, can be attributed to rapid expansion and effective gene flow across a more or less continuous niche due to disturbance tolerance. In contract, high diversification in the high altitude species, S. andina, can be explained by niche differentiation (IBE) and fragmentation (IBF) caused by the Andean uplift.