Rheumatoid arthritis is a disorder in which the joints swell and cause pain. Rheumatoid arthritis causes the body's own immune cells to attack the membrane of the joints. Which body system does rheumatoid arthritis affect?

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.