Match each rock with the correct description. 1. quartzite
2. limestone
3. shale
4. basalt
5. sandstone

1. made from a dissolved mineral called calcite
2. made of large grains of sand
3. made of very tiny grains of clay
4. formed from sandstone
5. fine-grained rock with few or no crystals

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.