Evolutionary taxonomy, evolutionary systematics or Darwinian classification is a branch of biological classification that seeks to classify organisms using a combination of phylogenetic relationship and overall similarity. This type of taxonomy considers taxa rather than single species, so that groups of species give rise to new groups. The concept found its current form in the modern evolutionary synthesis of the early 1940s.
Evolutionary taxonomy differs from strict pre-Darwinian Linnaean taxonomy, which produces orderly lists rather than trees. Also, unlike cladism which only maps phylogeny, evolutionary taxonomy also offer a biological classification system.[1] While in phylogeny where each taxon must consist of a single ancestral node and all its descendants, evolutionary taxonomy allows for groups to be excluded from their parent taxa (e.g. dinosaurs are not considered to include birds, but to have given rise to them), thus allowing for paraphyletic taxa.[2]
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Linnaean taxonomy was an established discipline when Darwin formulated his theory of evolution. The idea of the Linnaean taxonomy as translating into a sort of dendrogram of the Animal- and Plant Kingdoms was formulated toward the end of the 18th century, well before the On the Origin of Species was published.[3] Among early works exploring the idea of a transmutation of species was Erasmus Darwin's 1796 Zoönomia and Jean-Baptiste Lamarck's Philosophie Zoologique of 1809.[4] The idea was popularised in the Anglophone world by the speculative, but widely read Vestiges of the Natural History of Creation, published anonymously by Robert Chambers in 1844.[5]
With Darwin's theory, this thought got a theoretical basis, and Tree of Life representations became popular in scientific works. Very limited knowledge of the fossil record at the time hindered the drawing of specific inferences about the ancestors of modern groups. Both in Vestiges of the Natural History of Creation and On the Origin of Species, the ancestor remained largely a hypothetical species, and Darwin was primarily occupied with showing the principle, and very carefully refrained from speculating on relationship between living or fossil organisms, using theoretical examples only.[4]
One of the first fossil groups to be recognized was dinosaurs, formally named by Richard Owen in 1842. With Darwin's theory of evolution being known, Thomas Henry Huxley used the fossils of Archaeopteryx and Hesperornis to pronounce the birds descendants of the dinosaurs.[6] Thus, a group of extant animals could be tied to a fossil group. The resulting description, that of dinosaurs "giving rise to" or being "the ancestors of" birds, is the essential hallmark of evolutionary taxonomic thinking.
As more and more fossil groups were found and recognized in the late 19th and early 20th century, palaeontologists worked to understand the history of animals through the ages by linking together known groups[7] The Tree of life was slowly being mapped out, with fossil groups taking up their position in the tree as understanding increased. With the modern evolutionary synthesis of the early 1940s, an essentially modern understanding of evolution of the major groups was in place.[1]
These groups still retained their formal Linnaean taxonomic ranks, giving rise to a number of units that were paraphyletic, i.e. where the descendants were considered a part of the daughter group rather than that of the ancestral group. Particularly on the level of orders and classes, most of the traditional vertebrate systematic units are paraphyletic, representing natural evolutionary grades rather than clades.
The two approaches differ in the use of the word monophyletic. For evolutionary systematicists, monophyletic means only that a group derives from a single common ancestor included in the group, whereas for cladists it also means that the group includes all species descended from that group.[2] The term holophyletic has been proposed for the latter meaning.
The product of evolutionary systematics is a division according to Linnaean taxonomy (which can then be used to form tentative conclusions about phylogeny); the product of a cladistic classification is a cladogram, which can then be used to recommend a taxonomy.[8]
Cladistics collects character data only from the taxa being classified. It does not consider the inferred characters of ancestors.[8]
Evolutionary systematics also differs in method from cladistics. Cladistics involves collecting data and feeding it into a computer program. Evolutionary systematics involves a researcher following flexible guidelines which consider various kinds of evidence (which need not be represented as discrete alternatives).[8]
Other debates between evolutionary systematists and cladists are not about the underlying approach, but on details. One is whether there is a danger of artificial classifications when preparing a classification using molecular phylogeny based on only a single gene or part of a gene.[8] Another is whether it is sufficient to study DNA from chloroplasts, mitochondria, and ribosomes, as opposed to non-ribosomal nuclear DNA.[2]