Which of the following characteristics would NOT increase or decrease the fitness of the organisms that possess it? a. Certain mice in a population carry genes that cause their fur to be darker than the fur of other mice in the population that do not possess those genes. b. Certain fruit flies in a population carry genes that increase their ability to survive longer without food than other fruit flies in the population that do not possess that gene. c. Certain sterile mules in a population carry genes that allow them to carry heavier loads than other sterile mules in the population that do not possess those genes. d. Certain humans in a population carry genes that cause their eyesight to deteriorate at a more rapid rate than other humans in the population that do not possess those genes. e. Certain male birds in a population carry genes that increase the size of their tail relative to the size of the tails of the male birds in the population that do not possess those genes.