The anthrax-causing bacterium, Bacillus anthracis, produces few toxins. However, it can multiply readily and ultimately it invades the entire bloodstream. Such a bacterium, therefore, has qualities of invasiveness and toxigenicity.

Which of the following statements is true about the relationship between genetic variation and natural selection? a. genetic variation must be present in a population before natural selection can act on it. b. genetic variation arises in a population as a result of natural selection. c. natural selection can act on a population whether there is genetic variation within the population or not. d. after natural selection acts on a population, the amount of genetic variation in the population always increases.