Which environmental factor could lead the a decrease in genetic variation in a population of tuna

the calvin cycle takes place in the stroma of the chloroplast. carbon dioxide enters the stroma of the chloroplast to combine with the five-carbon compound called ribulose1, 5-biphosphate to form two molecules of a three-carbon compound called 3-phosphoglyceric acid. this reaction is catalyzed by an enzyme called ribulose bi-phosphate carboxylase.