Which one of the following criteria is necessary for natural selection to occur? a. individuals show no variation in fitness levels. b. individuals have the same adaptations. c. a struggle for existence is apparent. d. enough resources exist for individuals to survive and thrive.

answer: a) a compound microscope has a greater magnification ability than a simple microscope.

explanation:

if the type of the food available changes, then the frequency of beak types will change, because birds with beaks more suited to the available food will be more successful over time.