Final answer:
Changes in gene expression and small mutations can result in closely related species taking different developmental pathways, despite having the same sets of development-related genes.
Step-by-step explanation:
Changes in the genes that are important to embryonic development have been relatively minor because early embryos are very fragile and even small mutations can result in death. Additionally, differences in gene expression can create phenotypic differences, where different genes are turned on at different times during development and in different cells. This can result in closely related species taking different developmental pathways, despite having the same sets of development-related genes.