The fossil record shows a strong and persistent trend of decreased size of a mussel. what is the most likely cause of this phenomenon?

Directional selection

Explanation:

Directional selection or positive selection is an aspect of population genetics that favours an extreme phenotype more than other phenotypes, resulting in more of the allele frequency favouring the extreme phenotype.

Examples involve fossil records which prove a reduction in black bears sizes during interglacial periods of the ice ages and a rise in black bear sizes at glacial period in Europe.