Classification
Biological classification is the process of placiing living things into groups. This involves detailed study of the individuals in a species. We can then place the species in a category with other species that show a number of similar characteristics.
Some classifications are artifical and are made for our convenience. For example, in a wild flower guide the plants are grouped according to the colour of the flower.

Taxonomy
Taxonomy is the study of the principles behind classification. This means the study of the differences between the species. These differences can be used to classify species. Species are usually grouped according to their physical similarities. All species that look very similar are placed together. Species that look quite different are placed in separate groups.

Natural Classification
The basic unit of natural classification is the species. A species is a group of individual organisms that are very similar in appearance, anatomy, physiology, biochemistry and genetics. Individual members of a species will show variation. For example, all the varieties of dog, are members of the same species. Underneath the obvious visible differences, all dogs are very similar.
As all members of the species are very similar, we can consider them to be closely related. In the same way, different species that are very similar can be considered to be closely related. Two closely related species will be placed in a group together. Then closely related groups will be placed together in a larger group. In this way the whole of the living world can be organised into a series of ranked groups - a hierarchy.

Increasingly, modern classification has come to reflect the evolutionary distance between species. We can think of all living things as belonging to an evolutionary tree. Any two species living today have had a common ancestor at some point in the past. The time at which the two species started to evolve separately is a branch point on the tree. The common ancestor appears on the tree at that branch point. The more recent the common ancestor, the more closely related the two specices are - they have a short evolutionary distance.

Phylogeny
Phylogeny is the study of how closely different species are related. It reflects the evolutionary relationships of the species. We can see this in the evolutionary tree. Also the more recently in the past they shared a common ancestor. This is used as a basis for natural classification.
Using an evolutionary tree, we can see certain evolutionary relationships that indicate how closely related the species.

Humans and gorillas share many features and are closely related.
So we have a common ancestor in the recent past.
We can call humans and gorillas monophyletic - because they belong to the same phylogenetic group.
Humans and gorillas can be placed in the same taxonomic group
The thrush is more closely related to the snake than to the mammals.
We can see this because the common ancestor shared by the thrush and the snake is more recent than the common ancestor shared by the thrush and the mammals.
Therefore the thrush must be placed in a different group from the mammals.
Similarly the snake is more closely related to the thrush than to the frog or the trout.
All species can be considered can be considered monophyletic if they all evolved from the same species.

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Undergraduate Biology (Biodiversity and Evolution) Notiz am Classification and Taxonomy, erstellt von siobhan.quirk am 25/05/2013.