There are estimated to be more than 10 million species on Earth. Imagine if each of them had a name, but we had no classification system by which to organise them.
This would be like a library with ten million books and a card catalogue, but with the books arranged haphazardly on the shelves with no organising principle. It might be possible using the card catalogue to find any book, but it would not be possible to browse along a shelf to find similar or related books.
Taxonomists classify organisms to help organise knowledge about them, and to make it possible to understand their relationships. Accurately classifying species is as important as naming them; indeed, names and classifications in taxonomy are intimately related.
A classification of 'wading birds' in the order Charadriiformes. A classification is a system for capturing the relationships of species
Our understanding of classification has changed markedly during the history of taxonomy, with three distinct phases each characterised by distinctly different ideas about how best to classify the world's organisms.
Linnaeus, often called the 'father of taxonomy' developed a system of classification based on a few readily observable characteristics of the species he named. For example, all flowering plants were grouped according to the number of stamens and number of pistils (the female parts of a flower). Determining which group (order) a plant belonged to was a simple matter of counting its stamens and pistils, then turning to the appropriate page in Linnaeus' book documenting all the world's known plant species.
Carl Linnaeus, often called the 'father of taxonomy'. Linnaeus introduced many important elements of modern taxonomy including the naming system still used today. His attempts to classify the plants and animals he named were less successful: his system of classification based on a few easily observable features was largely superseded by the time of his death in 1778.
This proved convenient for a time, but even before Linnaeus' death it had became obvious to taxonomists and others that his classification system had serious flaws. Many cases were noted of two species that appeared very similar in all respects, but if one had five stamens and one had ten they would be classified in different orders. Linnaeus' system was equivalent to organising the books in a library by their colours rather than by their contents. It was handy, but reflected the patterns of variation in nature very poorly.
Linnaeus' classification system was soon replaced by one that reflected these patterns more closely. In this system, taxonomists began classifying species using their overall similarities and differences. The more alike two species were (taking into account as many characteristics as possible) the more closely they would be classified.
This system of classification began in earnest in the late 18th Century and continued to the middle of the 20th. Taxonomists during this time sought a 'natural' classification, one based on observable patterns in nature that everyone could agree on. In many ways this was a golden age in taxonomy. The great age of European exploration saw naturalists circle the globe collecting new and fascinating specimens from remote and exotic locations (such as Australia), and taxonomists (mostly in Europe and North America) studying and classifying them. The 'natural' system of classification seemed to work extremely well, and was almost universally adopted.
Joseph Banks as a young man. Banks and other European explorers collected many thousands of specimens of Australian plants and animals. These were classified, firstly by European and North American taxonomists and later by Australian-based taxonomists based at museums and herbaria.
However, two key insights, one in the middle of the 19th Century and one in the middle of the 20th, ushered in a new understanding of how best to classify organisms.
The first was the publication in 1859 of Darwin's great work On the Origin of Species by Means of Natural Selection. At first, Darwin's theory of evolution appeared to ground and amply justify the natural system of classification being used by taxonomists at that time. Evolution by descent with modification should mean that two organisms that share a close common ancestor should be more similar to each other than two organisms that share a more distant ancestor. A classification system based on overall similarities and differences should reflect evolutionary relatedness.
It took nearly a century, however, for a flaw in the system of 'natural' classification to be identified, by the German entomologist Will Hennig.
Hennig thought deeply about another of Darwin's innovations, the idea of the tree of life. Darwin had introduced this idea – that all living species could be arranged into something like a family tree – and it gripped the imagination of taxonomists. Classification became seen as reconstructing the tree of life of evolution.
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.
But Hennig identified a seemingly counterintuitive paradox in this idea: two species could be very closely related, but be more dissimilar to each other than two species that were more distantly related; conversely, distantly related species could be confusingly similar.
The reason for this is simple, but profound.
Imagine tracing the evolution of a group of species through time. If an ancestral species evolves into two species, the two descendants should be reasonably similar to each other, differing only slightly as each adapts to its particular environment and way of life. If one or both of these in turn evolves into two or more further species, again the descendants are likely to be fairly similar to each other. This could continue for some time, with an evolutionary lineage dividing into or budding off many new species, each of which will differ in some ways from all others, but all of which are likely to be fairly similar overall.
But imagine now that one of these species again splits into two, but this time one of the daughter species adapts to a very different environment. Perhaps the ancestor and all descendants so far have lived on land, but this one adapts to life in water. Or perhaps it moves from a rainforest environment, where all its relatives and ancestors have so far lived, to a much drier habitat. Or, perhaps it adapts to eating a very different prey, or being pollinated by a new type of pollinator, or avoiding a new type of predator. One species will now be living a very different life from all its relatives.
Species adapt to their circumstances, and the new species is likely to quite quickly evolve to look very different from its cousin or sister species.
Hennig realised that, in such situations, a taxonomist working under the established 'natural' system of classification would very likely classify this one species into a different group (genus, family etc) from its close relatives, because of its differences. But this would be paradoxical if a natural classification is intended to reflect the pattern of evolution. It would mean that two very closely related species (our unusual species and its more conventional-looking sister species), both descended from a single recent common ancestor, would be classified into different groups.
Almost all modern taxonomists agree that classifying species by their degree of evolutionary relatedness provides a more useful and rigorous classification system than classifying by their degree of overall similarity and difference. The way taxonomists describe this is that all taxa – genera, families etc. – should be monophyletic: that is, they should include all the descendants of a common ancestor, and represent a whole branch of the tree of life.
A good example of the old and new classification system comes from dinosaurs. Imagine a dinosaur, and the picture that comes to mind is a lumbering giant, perhaps an Apatosaurus, or Tyrannosaurus rex. Another thing that comes to mind is that all dinosaurs are extinct.
But our modern understanding is that this picture is incomplete and inaccurate. It's now clear from careful study of fossils that Tyrannosaurus rex is much more closely related to seagulls than it is to Apatosaurus. Dinosaurs did not all go extinct when an asteroid crashed into Mexico at the end of the Cretaceous period. Rather, some of them survived and continued to evolve, into present day birds. The blue wren fossicking for insects in your backyard is a bona fide dinosaur!
The dinosaur Malurus splendens, also known as the splendid fairy wren. Birds are a group of dinosaurs that survived a mass extinction event at the end of the Cretaceous period. The fearsome Tyrannosaurus rex, a species of theropod dinosaur, is evolutionarily more closely related to the splendid wren than it is to other classic dinosaurs such as the giant Apatosaurus. Classification can sometimes be surprising.