The genetic material is highly organised and the cells contain membrane bound organelles such as Chloroplasts and Mitochondria. The decendants of these cells together with the procaryotes make up all life on earth. So, when attempting to divide up the diversity of life into categories, this is the first division to make. Following this, the organisms can be divided into groups based upon their habitat e.g. freshwater, marine, terrestrial, or by their physical features. Classification Classification is the ordering of animals into groups, based on their similarity.
Taxonomy is the study of theoretical classification.
The main point all systems of classification rely on, is that groups of similar organisms have a common ancestor. This is the earliest form of classification and is based on the fossil record evidence, the degree of similarity between organisms and any other available evidence such as ecology and lifecycle. The problem with this approach is that there are many gaps in the fossil record and assumptions have to be made. There is also no standard definition of each taxonomic group and the classification method is not measurable. This provided a reasonable method of classification.