Variation in gene frequencies within populations can occur by chance rather than by natural selection. This is referred to as
Or
Random unidirectional change in allele frequencies that occur by chance in all population and especially in small populations is known as

Options

A. genetic flow
B. genetic drift
C. random mating
D. genetic load

Explanation

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks about the variation in gene frequencies within populations that occurs by chance rather than through natural selection. It also mentions a term for random unidirectional changes in allele frequencies, particularly in small populations.

2. **Identifying Key Concepts**: The key concepts here are:
- Gene frequencies
- Chance events
- Natural selection
- Small populations

3. **Explaining Gene Frequencies**: Gene frequencies refer to how often a particular allele appears in a population. These frequencies can change over time due to various factors, including natural selection and genetic drift.

4. **Natural Selection vs. Genetic Drift**: Natural selection is a process where alleles that confer advantageous traits increase in frequency because they enhance the survival and reproduction of organisms. In contrast, genetic drift refers to changes in allele frequencies that occur by chance, particularly in small populations.

5. **Defining Genetic Drift**: Genetic drift is defined as a random unidirectional change in allele frequencies that occurs by chance. It can lead to significant changes in a population's genetic makeup, especially when the population is small.

6. **Example of Genetic Drift**: An illustrative example is provided: If a population of beetles with red (A allele) and blue (B allele) coloration experiences a random event, such as an elephant stepping on them, the allele frequencies can change drastically. If more red beetles are killed by this event, the frequency of the A allele decreases, demonstrating genetic drift.

7. **Conclusion**: The correct answer to the question is "genetic drift," as it captures the essence of random changes in allele frequencies due to chance events, especially in small populations.

**Final Answer**: Genetic Drift

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