If a lake receives a large input of a limiting nutrient, which of these would happen first? a . an algal bloom occurs. b . algae begin to die and decomposers take over. c . nitrogen compounds are recycled. d . the concentration of oxygen drops below the necessary level.

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.