Small populations are prone to change in allelic frequencies which decrease genetic variability, this is explained by:

Options

A. Sewall-Wright effect
B. Genetic load
C. Genetic flow
D. Mutation

Answer

### Step-by-Step Solution:

1. **Understanding the Question**: The question states that small populations are prone to changes in allelic frequencies that decrease genetic variability. We need to identify the concept that explains this phenomenon.

2. **Analyzing the Options**:
- **Genetic Load**: This refers to the presence of unfavorable genetic materials in a population. It does not specifically relate to small populations and their changes in allelic frequencies. Therefore, this option is incorrect.

- **Gene Flow**: This is the transfer of genetic material between populations. High gene flow can equalize allele frequencies between populations, making them more genetically similar rather than decreasing variability. Hence, this option is also incorrect.

- **Mutation**: While mutations introduce new alleles into a population, they do not specifically explain the changes in allelic frequencies in small populations. Thus, this option is not correct.

3. **Identifying the Correct Concept**:
- **C. Wright Effect (Genetic Drift)**: This concept refers to the changes in allele frequencies that occur in small populations due to random sampling effects. In small populations, chance events can lead to the loss of genetic traits, thereby decreasing genetic variability. This effect is particularly pronounced in isolated populations where the gene pool is limited.

4. **Conclusion**: The correct answer to the question is the **C. Wright Effect**, which is a form of genetic drift that explains how small populations can experience significant changes in allelic frequencies, leading to decreased genetic variability.