Sky island’s age
The Hawaiian archipelago is well documented for the correlation between age of the islands and their colonization and radiation of various plants’ lineages, such as the Hawaiian lobeliads (Givnish et al. 2009). In contrast, the West-Central Africa sky archipelago does not have a simple chronological age from east to west (Suh et al. 2008). Nevertheless, the six sky islands that we studied have a clear progression. Their estimated ages run from old (West-Central Cameroon) to young mountains (Bioko and Mt. Cameroon). Our plastome phylogeographic interpretation indicated that L. columnariscolonized the older sky islands in mainland Cameroon after establishing South Bioko’s younger sky islands. Moreover, the South Bioko colonization predated North Bioko.
Overall, this result contradicts our sky island age hypothesis because Cameroon’s sky islands with estimated ages of 3.0, 22.0, and 31.0 Ma are older than Bioko’s sky islands (ca. 1.3 Ma) (Table 2). Our results suggest a biogeography history with an inverse progression concerning the age of the Afromontane sky islands.
Table 2. Elevation and estimated age of six sky islands of the Cameroon Volcanic Line