The fossil record shows that the average size of black bears was large during cold glacial periods and got smaller during warmer time periods. what is this an example of?

directional selection

Explanation:

Directional selection is the term used to describe a type of selection where a specific phenotype represents an allele that is advantageous for a given situation. This advantageous allele increases its frequency while this situation is occurring, decreasing the frequency afterwards. This allele need not be dominant.

It is common for this type of selection to occur in environmental changes. For this reason, we can consider the question above as a great example of directional selection. This is because the question above shows that the fossil record shows that the average size of black bears was large during cold glacial periods and decreased in warmer periods.