A Review of Taxonomic Concepts and Species Delimitation in Cycadales

Abstract

Taxonomic data is essential to advance the discovery and description of biodiversity, as well as the study of evolutionary processes. Emerging large-scale datasets and new methods of analysis have provided different approaches to describe biodiversity. Here, we present a review of the taxonomic history in Cycadales including an analysis of historical taxonomic concepts and approaches used for species delimitation. We examine the trends in the publication of new species following taxonomic works in books, journals and horticultural catalogues, monographic projects and floras where species treatments were published. In addition, we review the studies concerning species delimitations using the literature available in scientific journals appearing in the database ISI Web of Knowledge. The approaches used were discussed throughout all research focused on empirical and theoretical considerations in each study. We review the current state of the studies on causal processes that have given rise to the currently recognized diversity. The trend shows that taxonomic work on discovery and description of species has been intensive in the last 40 years culminating in 38.8% of binomials published. As a result, we consider the relevance of the monographs and floras for identification of species for other biological disciplines and the content of these contributions is compared and discussed. A total of six criteria (diagnosability, phenetic, phylogenetic, genotypic cluster, niche specialization and coalescent) were detected from the following three approaches to species delimitation within Cycadales: traditional, integrative taxonomy, and monophyletic. In all cases, the results from these species delimitations not only provided a taxonomic treatment or proposed a new species, but also supposedly clarified the other species involved as a result of the new taxonomic concept of the new species described. Most investigations of species delimitation used the traditional approach or a phenetic criteria. Finally, we discuss evolutionary studies on causal processes involved in cycad diversity. This is considered in the context of species delimitation as hypothesis testing for a successful evaluation of variation in both genetic and morphological understanding.