What is a likely explanation for a substantial change in body form between two closely related taxa?

The changes in the number, sequence, and regulation of developmental genes.

Explanation:

A taxa is the plural form of a taxon. A taxon in taxonomy is commonly referred to as a group of organisms in terms of population to form a specific unit. Example of a taxon is order, family, and class. A significant change in two similar taxa could be as a result in number or sequence change as well as a change in the genes.