|Life||The Phylogenetic Tree|
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Life on Earth
|Phylogeny and Systematics
Systematics ‒ History of ideas
Phylogenetic systematics (Cladistics)
Stratigraphy and phylogeny
A new approach
The central principle of evolutionary understanding is the Phylogenetic Tree. That is, evolution does not proceed in a simple straight line, but as a complex tree with various diverging, and sometimes also converging, branches going in many different directions.
This insight, that evolution is tree-like, constituted a huge shift in understanding that still has not totally overthrown the old linear thinking.
What came before the evolutionary tree was the Great chain of being. This evocative phrase was coined by historian of ideas Arthur O. Lovejoy in his study, called, what else, The Great Chain of Being, of the historical idea that all beings constitute a continuous series of forms an unbroken gradation from the Absolute (later, God) down through intermediate spiritual and material stages to formless matter. The premise was developed by Greek philosophers such as Plato (transcendent ideas), Aristotle (scala naturae), and Plotinus. In the Middle Ages it was the basis for both scholastic theology (ranking all of creation from dirt through to humans to angels) and feudal social stratification; it formed a central element in the Elizabethan understanding of the world still evident in Shakespeare's plays. It continued through 17th, 18th and 19th century Europe and North America, in an understanding of the universe as the highest good, complete and full (Lovejoy refers to this belief as "the Principle of Plenitude") in which every species of being has its perfect place and no species can ever become extinct (to do so would result in a gap in God's creation), and understanding this harmonious linear order of nature as a product of God's benign creative activity was a meaningful pursuit. From the end of the 18th century, with the "temporalization of the great chain of being" the meme persisted in evolutionary form (now as ascent rather than descent) through 19th century German Idealism and Naturphilosophie. Despite the rise of Darwinian evolution and a branching tree of life, the idea of a single line of ascent continued through mid 19th to early 20th century ideas of sociological evolution and social darwinism, and early to mid 20th century psychology. It even continues in some contemporary approaches such as Integral Theory. MAK111018
The extreme development of Great Chain of Being/Ladder of Nature thinking came with Swiss naturalist and spiritual philosopher Charles Bonnet (1720‒1793), who in his 1745 Traité d'insectologie traced the scale of nature in such detail that it became an absurdity. Hydra became a link between plants and animals, snails and slugs between molluscs and serpents, the ostrich, bat, and flying fox links between birds and mammals. However he also applied the Great Chain of Being to proto-evolutionary theories of ascent ‒ he believed that catastrophes such as Noah's flood brought about evolutionary change, and that after the next disaster, men would become angels, mammals would gain intelligence, and so on (Wikipedia). It became clear that nature could not be portrayed in a single dimension. The great German naturalist, Peter Simon Pallas (1741‒1811), in his Elenchus Zoophytorum (1766) showed that no linear scale can represent the mutual relations of organised beings; the branching tree, he said, is the appropriate metaphor. (see John S. Wilkins, The first use of a taxonomic tree). By the beginning of the 19th century, branching diagrams were used by the French botanist Augustin Augier in 1801, the French evolutionist Jean-Baptiste Lamarck (1744-1829), who produced the first branching tree of animals in his Philosophie Zoologique (1809) based on the Great Chain of Being, and the American geologist Edward Hitchcock (1763‒1864), who in 1840 published in his Elementary Geology, the first Tree of Life based on paleontology. Charles Darwin also drew abstract trees; Darwin's being the first evolutionary tree of life. It was Ernst Haeckel however who enthusiastically constructed several Trees of Life. Shown at the top of this page is his famous illustration published in The Evolution of Man (1879), which shows a Great Chain of Being model with Homo sapiens at the top, an image as iconic as the much misunderstood March of Progress, and which perhaps was the ultimate inspiration for the latter (although both go back ultimately to great chain of being / ladder of nature thinking). Although this would seem an anthropocentric step backwards in relation to his earlier but more contemporary-looking three-kingdom model, it should be remembered that for Haeckel, as for many 19th century evolutionists, humans were considered he pinnacle of evolution. Teilhard de Chardin serves as a 20th century example. Since then a number of sophisticated trees have been drawn. In evolutionary systematics, trees were generally spindle diagrams that mapped geological time (vertical axis) against taxonomic diversity (horizontal width) and emphasised monophyletic sensu Haeckel taxa (i.e. both monophyletic and paraphyletic groups). Sometimes width did not reflect diversity but was simply artistic license. For an example of such a tree see the diagram at evolutionary systematics. Supplanting evolutionary systematics in the 1980s, phylogenetic systematics placed great emphasise on tree diagrams, called cladograms, which are based either on gross morphology, molecular phylogeny, or both. Some of these diagrams can be incredibly detailed. MAK111018 .
Links: The Great Tree of Life diagram, artistic semi-stylised but still useful representation, by Leonard Eisenberg, evogeneao (evolutionary genealogy) website; The Tree of Life: Tangled Roots and Sexy Shoots ‒ Tracing the genetic pathway from the first Eukaryotes to Homo sapiens by Chris King (Biocosmology and Consciousness Research), huge "big picture" coverage; Wikipedia Tree of Life a valuable reference website (albeit still very incomplete, although some taxa, e.g. Agnatha, Ankylosauria, are well represented) with the an interactive presentation of the full "tree of life". Detailed references are supplied on each page about particular organisms. It is not entirely up to date with latest ideas; for example, the tree of Eutheria fails to reflect the recent classification into Laurasiatheria, Afrotheria, Euarchontoglires, and Xenarthra. Link (EvoWiki). MAK111018;
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content by MAK111018, edited RFVS111202