Final answer:
The closest evolutionary relatives to Mysticeti are the Odontoceti, with the hippopotamus being the closest living relative outside of the Cetacean group, due to a shared semi-aquatic ancestor.
Step-by-step explanation:
The Closest Evolutionary Relative to Mysticeti-
The Mysticeti, commonly known as baleen whales, are closely related to the Odontoceti, or toothed whales, which include species such as dolphins and killer whales. Both groups fall under the larger classification of Cetaceans. The closest living relatives to Cetaceans are the artiodactyls (even-toed ungulates), specifically the hippopotamus. This connection is supported by molecular and morphological evidence that indicates hippos and whales share a common semi-aquatic ancestor that lived approximately 40-50 million years ago.
It is important to mention that while hippos are the closest living relatives to Mysticeti, the actual closest relatives in terms of direct evolutionary lineage would be other whales within the Cetacean group, more specifically, their fellow subspecies under the infraorder Cetacea. The evolution of Cetaceans from land-dwelling ancestors to fully marine mammals is a remarkable example of adaptive radiation and convergent evolution, particularly as they adapted to life in the water, leading to the loss of hind limbs and modification of front limbs into flippers.