For evolution to occur in a population, which of the following must happen?

Options

A. The frequencies of some alleles in an organism's genotypes must change during its lifetime.
B. The frequencies of each allele in an organism's genotype must remain constant within its lifetime.
C. The frequencies of some alleles in a population's gene pool must change over successive generations.
D. The frequencies of each allele in a population's gene pool must remain constant over successive generations.

Explanation

To determine what must happen for evolution to occur in a population, we can analyze the options provided and understand the key concepts related to evolution.

### Step-by-Step Solution:

1. **Understanding Evolution**:
- Evolution refers to the change in the inherited characteristics of biological populations over successive generations. It is driven by changes in allele frequencies within a population.

2. **Analyzing the Options**:
- **Option 1**: "The frequency of some allele in organism genotype must change during its lifetime."
- This statement is incorrect because evolution is not about changes occurring within an individual's lifetime. Evolution occurs over generations, not within a single organism's life.

- **Option 2**: "The frequency of each allele in organism genotype must remain constant within lifetime."
- This statement is also incorrect. If allele frequencies remain constant within a lifetime, it implies no evolution is happening. Evolution requires changes in allele frequencies over time.

- **Option 3**: "The frequency of some allele in population gene pool must change over successive generations."
- This statement is correct. For evolution to occur, there must be changes in allele frequencies in the population's gene pool over generations. This is a fundamental principle of evolutionary biology.

- **Option 4**: "The frequencies of each allele in population gene pool must remain constant over successive generations."
- This statement is incorrect. If allele frequencies remain constant over generations, it indicates that no evolution is taking place.

3. **Conclusion**:
- Based on the analysis, the correct answer is **Option 3**: "The frequency of some allele in population gene pool must change over successive generations." This change is essential for evolution to occur.

### Final Answer:
The correct answer is **Option 3**.

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