Who proposed the theory of origin of species by natural selection?

Explanation

**Step-by-Step Solution:**

1. **Understand the Concept of Natural Selection:**
- Natural selection is a process where living organisms adapt to their changing environment. It is a mechanism of evolution.

2. **Identify the Key Elements of Natural Selection:**
- Organisms that are better adapted to their environment are more likely to survive and reproduce. These organisms are considered "fit."
- Conversely, those that are less adapted may not survive and can become extinct.
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