Which of the following is incorrect about the taxonomical aid keys?

The correct option is used for identification of plants and animals based on similarities only.

Let us break down the features of taxonomic keys step-by-step to understand why this statement is incorrect:
1. Definition and Purpose: A taxonomic key is an important taxonomical aid used by biologists for the identification of unknown plants and animals. This identification is done by comparing the characteristics of an unknown specimen with the known details of established taxa.
2. Contrasting Characters (Couplet and Lead): Keys are typically based on a pair of contrasting characters called a couplet. Out of the two contrasting choices, one is accepted while the other is rejected. Each individual statement within a key is called a lead, as it guides the user to the next logical step in the identification process.
3. Analytical Nature: Taxonomic keys are structured in a step-by-step, analytical format. By systematically making choices and eliminating options, the user can determine the correct identity of the organism. Therefore, keys are indeed analytical in nature.
4. Similarities and Dissimilarities: To successfully identify and group organisms, a key must look at both what makes them similar to certain groups (similarities) and what sets them apart (dissimilarities). Relying on similarities alone would make it impossible to differentiate between closely related species.

Hence, the statement that keys are used for identification based on "similarities only" is incorrect, making it the correct choice for this question.