Define atavism. Give one example of atavism.

Explanation

### Step-by-Step Text Solution:

1. **Definition of Atavism**:
Atavism is defined as the reappearance of a trait or organ that had disappeared in previous generations. It involves the re-functioning of a vestigial organ, which is an organ that has lost its original function through evolution.

2. **Explanation of the Process**:
Atavism occurs when a trait that was present in an ancestor re-emerges in an organism, even though it may have been absent in recent generations. This phenomenon can happen due to genetic mutations or the expression of dormant genes that were previously silenced.

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