NEWTON, ANGELA E.*, CYMON J. COX, EFRAIN DE LUNA, AND LARS HEDENAS. Department of Botany, Natural History Museum, London, SW7 5BD, UK, Duke University, Durham NC 27708, U.S.A., Instituto de Ecologia, A. C., Xalapa, Veracruz, Mexico and Swedish Museum of Natural History, Stockholm. - Evolutionary radiation of the pleurocarpous mosses: phylogenetic analyses incorporating morphological characters with rbcL, rps4 and trnL-trnF sequence data.
The three hypnobryalean orders of pleurocarpous mosses, Hookeriales,
Hypnales, and Leucodontales, are difficult to define using
morphological criteria, with "defining" characters of a
group often appearing elsewhere and species and genera repeatedly
shifting allegiances. Similarly, the circumscription and
relationships of the eubryalean pleurocarps, and their relationship to
the hypnobryalean pleurocarps and acrocarpous outgroup taxa, is also
problematic. Results of phylogenetic analyses of pleurocarpous mosses
using molecular sequence data, from the rbcL, trnL-trnF, rps4 and 18S
genes, have underscored this problem. In the current analysis, using
molecular characters, the hypnobryalean orders form a monophyletic
group with exemplars from the Hookeriales in a clade sister to
exemplars from the Hypnales and Leucodontales, but with these two
orders paraphyletic and poorly resolved. This pattern, seen also in
earlier studies, indicates a rapid evolutionary radiation, with very
short internodes that are difficult to resolve satisfactorily with the
molecular sequence data at hand. However, whereas molecular sequence
characters are generally thought to evolve in a regular,
"clock-like" fashion, morphological characters are thought
to evolve following a different pattern, with long periods of stasis
interspersed with periods of rapid change. Morphological characters
may therefore be able to resolve the relationships of taxa involved in
rapid radiation events. Many moss morphological features are
difficult to define as characters and states for cladistic analysis,
especially those "gestalt" characters that have played a
large part in taxonomic concepts. Other features show extensive and
complex variation that may represent several different characters.
Branching architecture and rhizoid morphology are two such complex
features that have been deconstructed to yield characters for
inclusion in cladistic analyses, and to provide a basis for study of
the evolution of these features.
Key words: evolutionary radiation, morphology, pleurocarpous mosses, sequence data