The challenges of classifying big genera such asIpomoea

Abstract

Big genera represent a significant proportion of the world's plants. However, comprehensive taxonomic and evolutionary studies of these genera are often complicated by their size and geographic spread. This paper explores the challenges faced in classifying these megadiverse plant groups consequent to the existing tension between diagnosability and increasing levels of resolution from molecular sequence data. We use recent examples from across angiosperms to illustrate how monophyly, diagnosability and completeness interplay with each other in attempts to classify several big genera and, specifically, the genus Ipomoea (Convolvulaceae). Ipomoea and the tribe Ipomoeeae have been the object of recent taxonomic and phylogenetic studies that highlight the limitations of previous attempts to classify the group, and show that the smaller segregate genera traditionally recognised in Ipomoeeae are nested within Ipomoea and are neither monophyletic nor diagnosable. We argue that existing classifications must be abandoned, and that recognising an expanded Ipomoea that incorporates all segregate genera of the Ipomoeeae is the most appropriate solution as it reconciles the properties of monophyly, diagnosability and completeness, and favours nomenclatural stability.

Rebuttal to “(2786) Proposal to change the conserved type of Ipomoea, nom. cons. (Convolvulaceae)”

“…(2020) are based on an earlier phylogenetic analysis (Eserman & al., 2014) that included only 27 species to represent more than 800 species. It is thus important to note that any re‐arrangements other than recognising an expanded, monophyletic Ipomoea as advocated by several authors for more than 20 years (e.g., Wilkin, 1999; Manos & al., 2001; Muñoz‐Rodríguez & al., 2019, 2022, 2023) would require more nomenclatural changes than those carried out by Muñoz‐Rodríguez & al. (2019).…”

“…), some authors regard the presence of the type of Ipomoea in the ‘Argyreiinae’ clade as an obstacle towards a most useful renewal of the re‐circumscription of the genera in tribe Ipomoeeae […].” In fact, we have never advocated a re‐circumscription of Ipomoeeae into smaller genera. In all our publications we have argued that such re‐circumscription is unnecessary and doomed to failure, as it is not possible to identify monophyletic and diagnosable taxa (genera, subgenera, sections) that include all species in Ipomoeeae (Muñoz‐Rodríguez & al., 2019, 2022, 2023; Wood & al., 2020). An expanded monophyletic Ipomoea is therefore the most appropriate solution for a large (c. 800–850 species) but not enormous genus compared to others such as Begonia , Carex , or Solanum .…”

“…As it stands, the proposal implies the continued acceptance and extension of a system of non‐monophyletic and non‐diagnosable genera nested within several hundred species of Ipomoea . All previous attempts to recognise segregate genera consistently failed to reconcile monophyly and diagnosability, and always left a large number of Ipomoea species unaccounted for (Wood & al., 2020; Muñoz‐Rodríguez & al., 2022, 2023). We are therefore sceptical of the assertion by this group of signatories – the majority of whom have little to no experience of taxonomic or systematic research on Ipomoea – that a satisfactory natural reclassification of these plants can be produced when this has not previously been possible, and they provide no such classification or basis for one.…”

“…(2020). However, this is not the case: the authors do not mention that the Old World clade includes more species of Ipomoea than species of all segregate genera combined, c. 230 vs c. 170 respectively, nor do they mention that all but one of the segregate genera are not monophyletic but intermingle with Ipomoea species (Muñoz‐Rodríguez & al., 2019, 2023). Only by using a poorly sampled and cherry‐picked phylogeny of Ipomoea in the broad sense, not including any “Argyreiinae” Ipomoea species (as by Simões & al., 2022), can an artificial split between Ipomoea and the current non‐monophyletic segregate genera be misleadingly portrayed.…”

Rebuttal to “(2943) Proposal to conserve Selaginella, nom. cons., (Selaginellaceae) with a conserved type”: An unwarranted and disruptive idea

“…(2023) consistently fail to reconcile monophyly and taxonomic diagnosability. This issue has also been pointed out as a major stumbling block for similar attempts to split equally large angiosperm genera such as Ipomoea L. (Muñoz‐Rodríguez & al., 2023a,b) and Miconia Ruiz & Pav. (Goldenberg & al., 2013; Michelangeli & al., 2016, 2022; Moraes & Goldenberg, 2017).…”