Botany · Ch 5 — Taxonomy and Systematic Botany
Cladistics
Cladistics
Cladistics (from the Greek klados, 'branch') is the method of classifying organisms into MONOPHYLETIC groups of a common ancestor, based on shared, derived (apomorphic) characters. These shared characters can be morphological (leaf, flower, fruit, seed), behavioural (e.g. nocturnal versus diurnal flower opening) or molecular (DNA or protein sequence). Its output is a cladogram - a tree-shaped diagram representing the best hypothesis of phylogenetic relationships; early cladograms were built mostly from morphological data, while today genetic-sequencing data and computational software are standard.
Cladistics recognises three group types. A Monophyletic group comprises ALL the descendants of a common ancestor - the only group type cladistics fully accepts. A Paraphyletic group includes an ancestor but NOT all of its descendants (e.g. the groupings labelled CB, CBD and ACB); this is occasionally tolerated for practical convenience (e.g. 'dicots', Sterculiaceae). A Polyphyletic group includes members from two different, unrelated lineages and is formally REJECTED by cladistics. …
What this figure shows. Three small branching tree (cladogram) diagrams contrasting the three group types cladistics recognises: (i) a Monophyletic group, where taxa A, B, C, D all descend from one common ancestor and all are included in the group; (ii) a Paraphyletic group (illustrated with the labelled example groupings CB, CBD and ACB), where an ancestor is included but not all of its descendants - e.g. only B, C, D are grouped while A, though descended from the same ancestor, is left out; (iii) a Polyphyletic group, where taxa such as W, X, Y, Z are grouped together despite arising from two different, unrelated lineag …