Why is the hierarchical scheme of classification arbitrary

Taxonomy is a term coined by Augustin Pyramus de Candolle in and is the study of the principles and methods of classification. The Linnaean System depends upon the hierarchical structure in which organisms are grouped. According to this system the animals belonging to the higher level, need not necessarily be similar to its other members, but the members of the lower levels are very similar and can reproduce.

The typical hierarchical system of classification is:. Note: You will see this system in use in the fossil images descriptions. He published the results of his travels as the Flora Lapponica, and later in published his famous Systema Naturae , which brought him a world wide reputation.

Carolus Linnaeus he loved Latin and used it for everything, including changing his name to the Latin version , is considered to be the founder of modern taxonomy. His work is the start of modern botanical and zoological classification. Like Aristotle before him, Linnaeus used logic in devising his classification scheme.

But he introduced a new idea; a hierarchy of nested groups within groups, a system sometimes called hierarchical ordering. An example of how this works is shown in the example below.

Place your cursor over the buttons beside the "2D World" to see how groups nest within groups. Division ONE. Division TWO. At first, Linnaeus tried to use a "natural" classification or taxonomic ordering, for example, he divided up all living organisms in to two major groups Kingdoms which he called 'plants' and 'animals'. All newly discovered creatures, therefore had to be first to be placed in one of these groups. It seemed "natural". Cnidarians, for example, had already split into the lineages we recognize as classes before echinoderms came onto the scene.

As a paper in Nature Ecology and Evolution pointed out, there is more divergence between those cnidarian classes — which include the Scyphozoa jellyfish , the Cubozoa box jellies , the Anthozoa sea anemones, corals, sea pens , and the highly diverse Hydrozoa — than between humans and sea urchins. Some biologists have argued that the definitions of phyla and other taxonomic ranks should be based more strictly on when these groups evolved.

Of course, considerable changes would need to be made to the currently recognized phyla: Either later-evolving groups would have to be lumped together, or lower ranks in older clades like the Cnidaria would need to be elevated to the rank of phylum. Even if phyla were defined on the basis of evolutionary timing, deciding which point in the evolutionary process to use would still be arbitrary and anthropogenic.

Despite the varying degrees of morphological divergence and the range of divergence times among phyla, many evolutionary biologists remain convinced that there is something unique about them.

This idea is pervasive in evolutionary developmental biology, for example, with many groups seeking to find an objective definition for phyla. In a Nature paper , for example, a group of researchers claimed to have found that long-sought defining feature. After examining developmental transcriptomes from 10 animals belonging to putatively different phyla, they concluded that unique signaling pathways in the middle of development can be used to objectively define phyla.

Notable differences between individual species within phyla are to be expected because each lineage developed independently over hundreds of millions of years. That tells us almost nothing, though, about how the lineages originally split. In the end, there simply seem to be no objective, consistent criteria by which to define a phylum — or any other rank, for that matter. The hierarchical nature does provide some meaningful information — so long as you stay within a group.

Unfortunately, scientists make apples-to-oranges comparisons based on Linnaean ranks all the time. For example, they may talk about rank-level characteristics, such as divergence at the genus level, that vary greatly between groups. Taxonomic labels are still merely labels — names devoid of units. Briggs, like many scientists, acknowledges that the Linnaean ranked system has flaws and can lead to errors.

But to do away with it, biologists would need a replacement. He and his colleagues have proposed an alternative system, the PhyloCode , to avoid these problems. Currently, taxonomic names are associated with particular ranks in the Linnaean hierarchy. More knowledge disrupted the taxonomy. That name would be more stable and enduringly meaningful because it could handle any new evolutionary connections that come to light.

The attachment to the Linnaean system seems to be largely driven by inertia. Throwing out ranks and defining life by some objective measure would be a big change, de Queiroz says, and no one seems ready to do that. Collins also acknowledges a lack of urgency.