Both molecules and compounds are made up of . Two or more atoms joined together by shared electrons form
. Substances made from two or more different elements are
.

What is the following best defines climate?

When a gasoline engine burns gasoline, what type of chemical reaction is occurring?

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.