Which statement correctly describes the effect of abiotic factors on population density? A. A change in abiotic factors always decreases the size of a population.
B. A change in abiotic factors always increases the size of a population.
C. A change in abiotic factors never impacts the size of a population.
D. A change in abiotic factors may increase or decrease the size of a population.

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.