Random non-directional change in allele frequencies that occurs by chance in all populations and especially in small populations is known as:

Options

A. Mutation
B. Migration
C. Natural selection
D. Genetic drift.

Answer

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks for a term that describes a random, non-directional change in allele frequencies that occurs by chance, particularly in small populations.

2. **Analyzing the Options**:
- **Mutation**: This refers to a sudden change in the DNA sequence. While mutations are random, they typically affect individuals rather than entire populations and are not the primary mechanism for changes in allele frequencies.
- **Migration**: This involves the movement of individuals from one population to another. Migration can affect allele frequencies but is not random or non-directional, as it involves specific movements of organisms.
- **Natural Selection**: This is a process where certain traits become more common in a population due to their advantages in a given environment. This process is directional, as it favors specific traits based on environmental pressures.
- **Genetic Drift**: This is a mechanism of evolution that involves random changes in allele frequencies, especially significant in small populations. It does not favor any particular direction and can lead to large changes over time due to chance events.

3. **Identifying the Correct Answer**: Based on the analysis of the options, genetic drift is the term that best fits the description of a random, non-directional change in allele frequencies that occurs by chance in all populations, particularly in small populations.

4. **Conclusion**: Therefore, the correct answer is **genetic drift**.

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