Final answer:
Substantial changes in body form between closely related taxa are most likely due to changes in developmental genes, including gene duplications and mutations that affect protein functions.
Step-by-step explanation:
A likely explanation for a substantial change in body form between two closely related taxa is A. changes in the number, sequence, and regulation of developmental genes. These changes can occur due to various genetic mechanisms such as gene duplication events, where an entire gene is added as an extra copy in the genome. This duplication allows one copy to undergo mutation, selection, or drift while the other copy continues its original function, potentially leading to new functions or features.
Additionally, the evolution of gene regulatory sequences like promoters can result in increased gene expression, contributing to changes in body form. Finally, substitution and frameshift mutations can lead to amino acid changes, altering the structure and function of proteins and, therefore, the traits of an organism.