At what level of grouping of organisms does evolution occur?

Explanation

**Step-by-Step Solution:**

1. **Understanding Evolution**: Begin by defining evolution. Evolution is a gradual process where small, sudden changes accumulate over time within a population of organisms.

2. **Identifying the Unit of Evolution**: The question asks for the level at which evolution occurs. The key term here is "unit of evolution," which refers to the basic level at which evolutionary changes can be observed.

3. **Role of Populations**: Evolution occurs within populations, which are groups of individuals of the same species that live in the same area and interbreed. A population contains a gene pool, which is the total collection of genes and their variants in that population.

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