The foundation for biological evolution or descent through modification is

 A)  changes in gene frequency in a population occur from one generation to the next.

The foundation of biological evolution is changes in gene frequency in a population occur from one generation to the next. Mutation, migration, genetic drift, and natural selection along with sexual reproduction result in genetic variation or changes in gene frequency.

~Gotten from USATestprep~

when atoms of different types of elements join together, they make molecules called compounds. water consists of compound molecules made up of 2 hydrogen atoms and 1 oxygen atom. this is why it's called h2o. water will always have 2 times the number of hydrogen atoms as oxygen atoms.