Monophyly or paraphyly--the taxonomy of Holcoglossum (Aeridinae: Orchidaceae).

Dirk Steinke,Xiaoguo Xiang,Weitao Jin,Hao Hu,Hailang Zhou,Yangjun Lai,Dezhu Li,Xiaohua Jin

doi:10.1371/journal.pone.0052050

Abstract

Recently, there have been a lot of intense debates about the acceptance/rejection of paraphyletic groups in biological classification. On the one hand, evolutionary classification states that similarity and common descent are two criteria for biological classification and paraphyletic groups are natural units of biological classification. On the other hand, cladistic classification considers that common descent is the only criterion in biological classification and monophyly should be strictly adhered to. Holcoglossum is used herein as a case to illustrate this problem. Although Holcoglossum is a small orchid genus of less than 20 species, there is little consensus about its generic circumscription since it was established, which leads to confusion in taxonomic treatments in the Aerides-Vanda group. Based on the analyses of molecular and morphological evidence, our results suggest that the clade comprising Holcoglossum s.s., Ascolabium, Penkimia and Ascocentrum himalaicum is strongly supported as a monophyly, and that the three taxa are nested within different subclades of Holcoglossum s.s. Thus, it is reasonable to recognize a monophyletic circumscription of Holcoglossum, which is also well supported by some vegetative and floral characters. The Holcoglossum s.l. would facilitate a better understanding of pollinator-driven floral divergence and vegetative stasis than a paraphyletic and narrowly defined genus.

Highlights

In the era of integrative taxonomy, there is more major congruence in the biological classification between cladistic classification and evolutionary classification except with regard to the acceptance/rejection of paraphyletic groups
Cladistic classification states the following: (1) only monophyletic groups in their strictest sense that are evidenced by synapomorphous characters are recognized in biological classification; (2) only species and clades are objective, and supraspecific taxa are terminals in cladistic classification [15,16]; (3) there are no objective criteria to circumscribe paraphyletic groups, and paraphyletic groups are artificial classes created by taxonomists to emphasize some particular characters or divergence [5,16,17,18] and (4) cladistic classification can be accommodated within the Linnaean system except for monotypic higher taxa and historic groups [19], with the former being neither paraphyletic nor monophyletic and the latter being dismissed [16,19]
Ascolabium, and Ascocentrum himalaicum are closer to Holcoglossum s.s. than to any other relatives (Table 1)

Summary

Introduction

In the era of integrative taxonomy, there is more major congruence in the biological classification between cladistic classification and evolutionary classification except with regard to the acceptance/rejection of paraphyletic groups. Evolutionary classification recognizing paraphyletic groups argues the following: (1) there are two criteria for biological classification, i.e., similarity and common descent [12]; (2) many species are paraphyletic [1,12,13,14]; (3) paraphyletic groups are natural transitional stages in the evolution of taxa and are natural units of biological classification [4]; (4) cladistic classification is incompatible with the Linnean hierarchy system [1,4,7], such as diachronous groups and (5) classification based only on common descent often fails to reflect divergence and natural selection. We used the taxonomy of Holcoglossum (Aeridinae, Orchidaceae) as a case to illustrate this problem here

Methods

Results

Discussion

Conclusion