Organisms vary considerably in genome size. Which of the following explains most of this variation?

A) differences in the number of protein-coding genes
B) differences in the number of ribosomal RNA genes
C) differences in the number of pseudogenes
D) differences in the number of mobile genetic elements

The primary factor contributing to the variation in genome size among different organisms is the difference in the number of mobile genetic elements. These elements, which include transposons and retrotransposons, can replicate within the genome and significantly increase its size. Unlike protein-coding genes, ribosomal RNA genes, and pseudogenes, the quantity of mobile genetic elements can vary greatly between species, leading to substantial differences in genome size.