Correct Option: B. Mutation and recombination Detailed Explanation: Genetic variation within a population is crucial for evolution and adaptation. The primary factors responsible for this variation are mutation and recombination. Let’s break down these concepts:

1. Mutation:
2. A mutation is a change in the DNA sequence of an organism. These changes can occur due to various reasons, such as errors during DNA replication, exposure to radiation, or chemical influences.
3. Mutations can be beneficial, neutral, or harmful. Beneficial mutations may provide an advantage in survival or reproduction, while harmful mutations can lead to diseases or reduced fitness.
4. Importantly, mutations are the original source of genetic diversity. Without mutations, all individuals in a population would be genetically identical, limiting the potential for evolution.
5. Recombination:
6. Recombination occurs during sexual reproduction when genetic material is exchanged between paired chromosomes. This process creates new combinations of alleles (gene variants) in the offspring.
7. During meiosis, homologous chromosomes can undergo crossing over, where segments of DNA are swapped between them. This shuffling of genes results in offspring that have a unique combination of traits, contributing to genetic diversity.
8. Recombination allows for the mixing of alleles from two parents, which can lead to new traits that may be advantageous in changing environments.

Together, mutation and recombination are the primary mechanisms that introduce and maintain genetic variation within a population, enabling evolution through natural selection. Why Other Options Are Incorrect: A. Environmental changes: - While environmental changes can influence which traits are advantageous and thus affect the survival of individuals, they do not directly create genetic variation. Instead, they act on the existing genetic variation within a population. Therefore, environmental changes are not a source of genetic diversity but rather a selective pressure. C. Natural selection: - Natural selection is a process that acts on existing genetic variation, favoring individuals with traits that enhance survival and reproduction. It does not create new genetic variation; rather, it shapes the frequency of existing variations in a population. Thus, while natural selection is crucial for evolution, it is not the primary source of genetic variation. D. Geographic isolation: - Geographic isolation can lead to speciation by preventing gene flow between populations, allowing them to evolve independently. However, it does not create genetic variation itself. Instead, it can preserve and amplify existing variations by separating populations, which may lead to divergent evolution over time. Summary of Key Points:

• Mutation introduces new genetic variations through changes in DNA sequences.
• Recombination mixes genetic material during sexual reproduction, creating diverse offspring.
• Natural selection and environmental changes act on existing variations but do not create them.
• Geographic isolation can lead to speciation but does not generate genetic diversity.

Understanding these concepts is essential for grasping how populations evolve and adapt over time.