Which is not an example of cell differentiation? a. a leaf cell divides as a plant leaf grows. b. a fertilized egg develops limbs and a head. c. stem cells from bone marrow develop into blood cells. d. bone marrow cells are used to repair damage to heart tissue?

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.