Which of the following is an example of necessary chemical reaction in a living organism?

In what part of the body does the most muscle activity happen?

The relationship between intron and exon

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.