Final answer:
Sexual reproduction is an activity that increases genetic variation within an earthworm population by combining genetic material from two different parents. Mutation also contributes to genetic variation by introducing new traits that natural selection can act upon.
Step-by-step explanation:
An activity that increases the genetic variation within a population of earthworms is sexual reproduction. This process allows for the mixing of genetic material from two different parents, resulting in offspring with a unique combination of genes. This genetic mixing increases diversity within the population, which can lead to a greater ability to adapt to environmental changes and resist diseases. In earthworms, which reproduce sexually, the combination of chromosomes from each parent during fertilization creates this genetic variation. This diversity is crucial for the population's adaptability and long-term survival.
Another source of genetic variation is mutation, a permanent change in the DNA sequence that can lead to new traits in the population. While a mutation can have various impacts on an organism's appearance or phenotype, it is the ultimate source of new genetic material that, through natural selection, can be propagated or suppressed based on its success in the environment.