Evolutionary history and systematics of Acer section Acer - a case study of low-level phylogenetics

The phylogenetic and systematic position of all species of Acer section Acer from North America, East Asia, and western Eurasia are evaluated using various splits-based networks (distance networks, bipartition networks), ITS motif analysis, and morphology. Molecular analyses are based on 276 ITS clones obtained from 101 specimens collected mainly from natural stands. The large sample size ensures to cover sufficiently inter- and intraspecific ITS variability of this group. Formerly recognised species are generally supported by ITS data and morphology; the combination of molecular (ITS) and morphological criteria allows defining seven (supraspecific) taxonomic groups prior to a phylogenetic reconstruction. Phylogenetic signals captured in modern ITS sequences are partly incompatible but clearly suggest that Acer section Acer underwent three major radiations. Horizontal gene flow is indicated between ancestors of extant taxa that are isolated at present times. The level of ITS derivation can be estimated and corresponds to levels of morphological differentiation and (palaeo-) biogeographical patterns. Based on our results we question the potential of cladistic approaches to infer low-level evolution in an adequate manner and demonstrate that speciation in members of Acer section Acer is not generally linked to cladogenesis. The data and methodologies provided here allow to trace pathways of low-level evolution and to analyse such data sets with a less restricted (non-dichotomous) dynamic concept.