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