Which is not applicable to the biological species concept?

Options

A. Hybridisation
B. Natural population
C. Reproductive isolation
D. Gene pool

Answer

To determine which option is not applicable to the biological species concept, we need to analyze each option provided: hybridization, natural population, reproductive isolation, and gene pool.

### Step-by-Step Solution:

1. **Understanding the Biological Species Concept**: The biological species concept defines a species as a group of individuals that can interbreed and produce fertile offspring in a natural environment, while being reproductively isolated from other such groups.

2. **Analyzing Each Option**:
- **Hybridization**: This refers to the process where two different species breed to produce hybrid offspring. In many cases, hybrids may not be able to reproduce or interbreed with either parent species, which contradicts the idea of a species being a group of interbreeding individuals. Therefore, hybridization is not applicable to the biological species concept.

- **Natural Population**: This refers to a group of individuals of the same species that live in the same area and can interbreed. This concept is fundamental to the biological species concept.

- **Reproductive Isolation**: This is a key component of the biological species concept. It refers to the mechanisms that prevent different species from interbreeding, thus maintaining species boundaries.

- **Gene Pool**: This refers to the total genetic diversity found within a species. The concept of a gene pool is relevant to the biological species concept as it encompasses the genetic variation that can be shared among interbreeding individuals of a species.

3. **Conclusion**: Based on the analysis, hybridization does not align with the biological species concept because it involves the mixing of different species, which can lead to offspring that do not fit the definition of a species under this concept. Therefore, the answer is **hybridization**.

### Final Answer:
**Hybridization** is not applicable to the biological species concept.