What is the term for a change in the relative frequencies of alleles in a population over several generations?

Microevolution

Explanation:

Change in allele frequencies through time occurs via evolutionary mechanisms: mutation, natural (and artificial) selection, genetic drift, gene flow (migration).

Macroevolution also represents the change in allele frequencies but on larger time scale: while microevolution is at species level, macroevolution  refer to all levels above species.

when you breathe in, or inhale, your diaphragm contracts (tightens) and moves downward. this increases the space in your chest cavity, into which your lungs expand. the intercostal muscles between your ribs also enlarge the chest cavity. they contract to pull your rib cage both upward and outward when you inhale.