Questions: An organism's fitness depends on its - physical size - ability to swap genetic material with other organisms - population - ability to mutate - ability to survive and reproduce

The answer is the last one: ability to survive and reproduce.

Explanation for each option:

1. Physical size: While physical size can influence an organism's fitness in certain environments, it is not a direct measure of fitness. Fitness is more about the organism's ability to pass on its genes to the next generation.

2. Ability to swap genetic material with other organisms: This ability, such as through sexual reproduction or horizontal gene transfer, can contribute to genetic diversity and adaptability, but it is not the sole determinant of fitness.

3. Population: The size of a population can affect genetic diversity and survival chances, but fitness is typically considered at the level of individual organisms rather than populations.

4. Ability to mutate: Mutation can introduce genetic variation, which is important for evolution, but not all mutations are beneficial. Fitness is more about the successful reproduction of beneficial traits.

5. Ability to survive and reproduce: This is the correct answer. Fitness in evolutionary biology is defined as an organism's ability to survive, reproduce, and pass on its genes to the next generation. This is the most direct measure of an organism's evolutionary success.

In summary, an organism's fitness is primarily determined by its ability to survive and reproduce, ensuring the continuation of its genetic material in future generations.