Taxonomy and phylogeny  (Page 2/6)

There are an estimated 8-9 millions species on our planet. Every organism that has been described fits somewhere in this classification scheme. You can explore this in much more detail at the Tree of Life website . There you will find information on the thousands of species that have been described and named, as well as their currently assigned place in the taxonomic scheme. It is also worth pointing out that the basic unit of this scheme, the species, is not a well-defined term at all. There are multiple definitions of species that you can find with just a minimal bit of effort, and none of them is entirely satisfactory. Just as we learned about the definition of life, it is sometimes difficult to pin down an exact point on a hierarchy. As you will see from the material below, the problems with a definition of species might be related to the fact that evolution is ongoing, and that some groups of organisms are at different points on an evolutionary path at the present time.

However, if we ignore the vexing issue of figuring out a widely-accepted definition for “species”, it is clear that a system based on this unit has several advantages. One of the immediate advantages of the Linnean system was that it allowed biologists to bypass the often confusing different common names that are used in various parts of the world, or in various languages. For example, the animal that is known as a moose in North America is confusingly called an elk in Europe. But when you refer to this creature by its scientific name ( Alces alces ), there is no confusion. That is why scientific papers always include the two term scientific name for a species. The hierarchical nature of the Linnean scheme is also useful, allowing us to easily understand the levels of similarity at different levels in the scheme. For example, organisms in the same genus can be very similar to each other, and may even be difficult to tell apart unless you are an expert. The similarity becomes less apparent for higher taxa; the class Aves includes that Bald Eagle mentioned above as well as your pet parakeet, which may screech like an eagle, but which is otherwise not very similar to an eagle at all.

The original Linnean taxonomy was based, as noted above, primarily on external morphology. It also relied on the obvious fact that organisms are related to each other in many ways, and that some seem more closely related than others. However, a system based solely on morphological similarities and differences can lead to some interesting classification errors. For example, Linneaus had one Phylum named Vermes (Latin for “worms”). This obsolete taxon included animals that we now classify with the mollusks, others that we now classify with the vertebrates, and still others that we now classify with crustaceans, as well as those that are still classified as worms. This reinforces two aspects of taxonomy that are important to remember. The first is that taxonomic classification schemes are merely hypotheses, and should be discarded or modified if newly obtained information is not consistent with that hypothesis. The second is that classification schemes based only on one type of information (in this case, external morphology) can be quite mistaken. Using more than one type of information will lead to better and more stable taxonomies.