Pteridological Section/AFS
Barrington, David S. [1].
Breaking with tradition: recent change in evolutionary mode among tropical American Polystichum(Dryopteridaceae).
ONGOING phylogenetic study of the fern genus Polystichum using three chloroplast sequences is revealing the broad history of the genus. Based on a sample of about one half of the genus, a single monophyletic lineage appears to comprise all of the diverse array of about 60 species in tropical America�and none of the remaining 200 species. The limestone-loving, often once-pinnate species of the Greater Antilles appear to have diverged from the common ancestor of the largely continental lineages early in the history of the genus in the neotropics. A second early event was the divergence of the finely divided P. speciosissimum of northern Latin America from the remainder of the continental lineage. More recently, the genus on the mainland has diverged into two groups, one of which � including all of the species with indusia � has all but one of its species with restricted ranges endemic to Mexico and Guatemala. In contrast, the other lineage, which includes all of the exindusiate species but one � includes only two species that are found outside the northern and central Andes. Thus, much of the Neotropical history of Polystichum has been characterized by local speciation events, not divergence following vicariance or long-distance dispersal. However, several species appear to have undergone dramatic range expansions in the recent past. For instance, the common, widespread Polystichum platyphyllum � a distinctive bulb-bearing species with long-attenuate leaves from stream edges at low elevations, is sister to an alliance of species restricted to high forests and p�ramos in the Central Andes. This pattern suggests that recent (Pleistocene) evolution of the dramatic changes in morphology and habitat preference characterizing P. platyphyllum have led to its dramatic range expansion.
1 - University of Vermont, Department of Botany, 109 Carrigan Drive, Burlington, Vermont, 05405-0086, USA
Polystichum
neotropical.
Location: 121/Tehama