Which of the following statements about the ice ages is true? a. An ice age will never come again. b. The causes of ice ages are well known. c. During the ice ages, icy glaciers covered much of the continents. d. There were no living organisms during the ice ages.

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.