Athena Review Image Archive ™
Cladistics
Potrait of Willi Hennig (photo: 1971)
Cladistics is a system of classification initiated by Willi Hennig, a Swiss entomologist or specialist in insect biology. His 1966 book Phylogenetic Systematics paved the way for the widespread adoption of the cladistic method of classification in biology. It is intended to be used for deciphering and describing the closest evolutionary relations of species, both extant and extinct, though the extensive comparison of anatomical traits called characters.
The goal of the method is to create phylogenies by arranging taxa by shared traits or characters into clades, or monophyletic groups where all members have the same ancestors. This relies on computer grouping programs to create branching or tree diagrams by nearest-neighbor and maximum parsimony methods. The phylogenies, or relationship trees, show presence or absence of common ancestors between species and groups of species called clades ("branches").
The comparative methods used by cladistics involve, first, the selection of various (sometimes hundreds) of anatomical traits or characters thought to be diagnostic for the species under consideration, and the listing of these in tables, for as many species as can be relevantly compared. These trait tables are then analyzed through computer programs such as PAUP, which perform complex pattern-grouping and parsimony analysis to create “nearest neighbor” tree diagrams. These are considered to reflect the probable closest relations between taxa, which can then be expressed in cladograms.
Although
not without its critics, cladistics at present is the predominant
method of classifying both fossil and extant taxa of organisms.
Cladistics has fundamentally different goals than the hierarchical
ordering of Linnaean classification, or than a third approach known as
evolutionary systematics.
Cladistics has its own terminology, which is quite distinct from that of the Linnaean system (see Benton 2000 for a detailed analysis). The relationship trees or “cladograms” aim to show clades or branches called "sister groups," of which every member shares a common ancester, and avoids portraying long term groupings such as classes or orders. The goal of requiring every member of a clade to share the same common ancestor (called a “monophyletic” group) also, in practical terms, requires a constant mode of revisionism by those writing up these findings. Every time a significant new fossil is discovered, it may prompt reorganization of the entire “clade” along with all neighboring branches (Benton 2000). While the revisions may be radical and improvisational, however, the digital methodology can be rather rigid. A seemingly problematic aspect, related to the standardization sought via using computer grouping methods, is the dependence on discrete traits which are either present or absent.
References
Benton, M.J. 2000, Stems, nodes, crown clades, and rank-free lists: is Linnaeus dead? Biol. Rev. 75: pp. 633-648
Hennig, W. 1966. Phylogenetic systematics. University of Illinois Press, Urbana
Copyright © 1996-2020 Rust Family Foundation (All Rights Reserved).