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 chemicals.
3. Mutations can be beneficial, harmful, or neutral. Beneficial mutations may provide an advantage in survival or reproduction, while harmful mutations can lead to diseases or reduced fitness. Neutral mutations do not affect the organism's fitness.
4. Importantly, mutations introduce new alleles (variations of a gene) into a population, which is a key source of genetic diversity.
5. Recombination:
6. Recombination occurs during sexual reproduction when genetic material from two parents is combined to produce offspring. This process happens during meiosis, where homologous chromosomes exchange segments of DNA in a process called crossing over.
7. This shuffling of genes results in offspring that have a unique combination of traits, contributing to genetic variation. Each offspring inherits a different mix of alleles from its parents, leading to a diverse gene pool.

Together, mutation and recombination create a rich tapestry of genetic diversity within a population, which is essential for evolution. This variation allows populations to adapt to changing environments and can lead to the emergence of new species over time. Why Other Options Are Incorrect: A. Environmental changes: - While environmental changes can influence which traits are advantageous (and thus affect natural selection), they do not directly create genetic variation. Environmental factors can select for or against certain traits but do not introduce new genetic material into the population. C. Natural selection: - Natural selection is a process that acts on existing genetic variation within a population. It favors individuals with traits that provide a survival or reproductive advantage. However, it does not create new genetic variation; it merely selects for the traits that are already present due to mutations and recombination. D. Population size: - Population size can influence genetic variation through genetic drift, which is a random process that can lead to changes in allele frequencies, especially in small populations. However, it does not create genetic variation. Larger populations tend to maintain more genetic diversity, but the source of that diversity is still mutations and recombination. Summary of Key Points:

• Genetic variation is primarily introduced through mutation and recombination.
• Mutations create new alleles, while recombination mixes existing alleles during sexual reproduction.
• Natural selection and environmental changes act on existing variation but do not create it.
• Population size can affect genetic diversity but is not a source of genetic variation itself.

This understanding of genetic variation is fundamental in fields such as evolutionary biology, conservation genetics, and medicine, as it underpins the adaptability and resilience of species in changing environments.