Theory:

Based on the various theories discussed above, the evolutionary relationships and classification of living organisms can be done.

Recall Diversity in living organisms from class 9, where we have discussed the hierarchies of characteristics between species.
What is meant by characteristic?
The particular feature or a specific function of an organism or an object is known as its characteristic.
Example:
The fact that humans have four limbs and plants perform photosynthesis are examples of acharacteristic.
Some of the characteristics that are used in the classification of living organisms are as follows:
  • Classification based on the presence and absence of membrane-bound cell organelles, i.e., whether an organism is a eukaryote or prokaryote. As the eukaryotes have a cell with a well-defined nucleus, the cellular processes occur efficiently in isolation. These cells can also participate in forming a multicellular organism. Thus nucleus serves as a basic characteristic of classification.
  • Classification based on the number of cells, i.e., whether an organism is unicellular or multicellular. In multicellular organisms, all the cells are not identical. Groups of cells carry out specialised functions. This leads to the distinction of body designs of organisms.
  • Based on whether an organism can prepare its food or not, the organisms are classified into autotrophs and heterotrophs. The organisms which can prepare their food are known asautotrophs, and the organisms which cannot prepare and depend on other organisms are known as heterotrophs.
  • Based on the development of the body, their parts and organs which perform different functions.
As a result, the more traits two species have, the closer they are related. Furthermore, the more closely they are related, the more recently they have had a common ancestor.
These relationships can be well explained with the help of the following example.
A brother and sister have a close relationship. They share common ancestors from the previous generation, notably their parents. A girl and her first cousin are related, but not as close as a girl and her brother. This is because cousins share common ancestors, their grandparents, who are in the second generation before them, rather than the first. We now understand that species classification is a reflection of their evolutionary relationship.
Small groups of species with recently shared ancestors can be formed, followed by super-groups of these groups with more distant common ancestors, and so on. In principle, we may keep moving backwards in time until we reach the concept of a single species at the beginning of evolution. If this is the case, the non-living matter must have given rise to life at some point in the earth's history. There are numerous theories as to why this occurred.
All the living organisms found on the earth are identified and categorised based on their body forms and functions. The impact of specific characteristics on the body design is more when compared to the others. When one body design comes into play, it shapes all the effects of the other subsequent design changes, as it already exists. Thus, the earlier characteristics are likely to be essential than the later ones.