Final answer:
Cetacean evolution involves a series of gradual adaptations from land-dwelling ancestors to modern aquatic whales and dolphins. No living species directly bridge this evolutionary gap, but numerous fossils provide a clear transitional record. The process was gradual, not sudden or saltatory.
Step-by-step explanation:
In cetacean evolution, there are not currently any living species that serve as intermediate forms or 'missing links' between land-dwelling ancestors and modern whales and dolphins. However, fossil evidence has provided a detailed picture of how these animals transitioned back to aquatic lifestyles. The process of cetacean evolution involved a series of gradual adaptations over millions of years, rather than sudden leaps or saltation. One of the most well-known transitional fossils is Pakicetus, which indicates early cetaceans had significant ties to land-dwelling even-toed ungulates. Similarly, other fossils like Ambulocetus and Rodhocetus help to bridge the gap between land mammals and modern cetaceans, showing progressive adaptations for aquatic life, such as changes in limb structure and the development of tail flukes.
Cetaceans are related to other artiodactyls (even-toed ungulates), and share a common ancestry with the hippopotamus, suggesting a divergent evolution from a shared terrestrial ancestor. This indicates that cetacean evolution was a complex process involving gradual anatomical changes, such as the modification of limbs into flippers, the transition of the nostrils into a blowhole, and the streamlining of the body for efficient swimming. The evidence does not support the idea of saltatory evolution, which would imply abrupt changes between species without transitional forms.