Introducing
Taxonomy
Evolution, phylogenies and the tree of life
Imagine drawing a family tree of all the species on Earth, including all species alive today and all ancestors from the first living organism. This is one of the great tasks of taxonomists and biosystematists.
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The tree of life – the family tree or 'phylogeny' of all organisms – is the greatest single organising principle of biology and biodiversity.
We are confident that all living species are related and are descended from a single, long-extinct ancestor. All the features of organisms – the way they look, the things they do, the way they live their lives – are the result of patterns of descent from extinct ancestors, and the continual evolution of new features, traits and innovations. As the great biologist Theodosius Dobzhansky once said, 'nothing in biology makes sense except in the light of evolution', and phylogenies are our way to understand and represent evolution.
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This early attempt to sketch the shape of the tree of life, a branching 'family tree' showing the evolutionary relationships of all organisms, is by the German taxonomist. artist and philosopher Ernst Haeckel. Haeckel was an early supporter and champion of Darwin's theory of evolution. His Kunstformen der natur is a masterpiece of scientific illustration. Modern trees of life (phylogenies) are more rigorous, detailed and sophisticated than Haeckel's.
Like family trees, phylogenies are usually drawn as branching diagrams, although in the case of a phylogeny species usually have only one 'parent' species. One dominant pattern of evolution is the splitting of species into new species, and a phylogeny is a representation of the historical pattern of species splitting.
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A recent phylogeny of kangaroos and wallabies (Macropus and Wallabia), showing the inferred way the various modern species have evolved and hence are related. This phylogeny suggests, among other things, that the wallaroo (M. robustus) and red kangaroo (M. rufus) are both the descendents of a recent ancestor, and that the swamp or black wallaby (M. bicolor) should be classified in the genus Macropus.
Redrawn from https://www.nature.com/articles/s41598-017-16148-0
In modern taxonomy, phylogenies are also the basis of classification. Since the mid-20th century, taxonomists have been working to ensure that our system of classification faithfully reflects the history of evolution, as this provides the most useful, predictive, and scientifically rigorous system of classification and naming.
There are two main ways to infer the pattern of evolution of species. One is by studying fossils. In a few groups of organisms, the fossil record is complete enough that it's possible to study evolution and speciation through time directly.
However, for most groups of organisms the fossil record is very incomplete or virtually non-existent. In these cases taxonomists use a different approach, instead inferring the likely pattern of evolution from the patterns of variation in living organisms. Powerful mathematical methods have been developed for doing this and testing the results, providing confidence that even without fossils we can learn about how organisms evolved. Genetic evidence is proving particularly effective in this respect: carefully analysing the DNA sequences of living species and using phylogenetic methods to infer their relationships has allowed, for the first time, an accurate and testable reconstruction of the whole tree of life, providing an exquisitely detailed and comprehensive view of the relationships of all living species, from bacteria to blue whales and everything in between.
This astonishing diagram is a draft phylogeny of all life, including all major groups of organisms from bacteria and the bacteria-like archaea to plants, fungi and animals. While this tree is drawn as a circle to save space, it is still read as a branching diagram – imagine straightening it out and it will be in the same form as the phylogeny of kangaroos and wallabies above. The kangaroo and wallaby phylogeny will be one tiny, invisible branch in the blue (metazoa) part of this tree.
Source: Credit: opentreeoflife.org
