This does not affect the argument. The researcher hypothesizes that guinea pigs and mice stem from separate ancestors based on her finding that they have dissimilar genetic patterns—the researcher does not make an argument about rodents broadly.

This weakens the argument. It attacks the researcher’s assumption that dissimilarity in genetic patterns is enough to conclude two species lack a single common ancestor by noting that some species who do share an ancestor also lack similar genetic patterns.

This does not affect the argument. The researcher only claims that the genetic difference between mice and guinea pigs is as great as between mice and “some” nonrodent species.

This does not affect the argument, which is that given their dissimilar genetic patterns, mice and guinea pigs do not share a single common ancestor at all. The recency of a common ancestor is irrelevant.

This does not affect the argument, which already notes that guinea pigs and mice share similar physical forms.