How would you choose between two characteristics to be used for developing a hierarchy in classification?

For developing a hierarchy of classification, we choose the fundamental characteristic among several other characteristics. These fundamental characteristics are independent of any other characteristics in their effects on the form and function of the organism. The characteristics in the next level would be dependent on the previous one and would decide the variety in the next level. In this way, we can build up a whole hierarchy of mutually related characteristics to be used for classification. For example, plants differ from animals in many ways such as body design, photosynthesis, the absence of locomotion, chloroplasts, cell wall, etc. But, out of these, only locomotion is considered as the basic or fundamental feature that is used to distinguish between plants and animals. This is because the absence of locomotion in plants gave rise to many structural changes such as the presence of a cell wall for protection, and the presence of chloroplast for photosynthesis (as they cannot move around in search of food like animals). Thus, all these features are a result of locomotion. Therefore, locomotion is considered to be a fundamental characteristic. By choosing the basic or fundamental characteristic, we can make broad divisions in living organisms as the next level of characteristic is dependent on these. This goes on to form a hierarchy of characteristics.

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Chapter 7. Diversity in Living Organisms | CBSE Class 9th Science