Hippopotamidae
The suggested subspecies were never widely used or validated by field biologists; the described morphological differences were small enough that they could have resulted from simple variation in nonrepresentative samples.[8]:2 Genetic analyses have tested the existence of three of these putative subspecies. A study examining mitochondrial DNA from skin biopsies taken from 13 sampling locations, considered genetic diversity and structure among hippo populations across the continent. The authors found low, but significant, genetic differentiation among H. a. amphibius, H. a. capensis, and H. a. kiboko. Neither H. a. tschadensis nor H. a. constrictus has been tested.[9][10]
Evolution
Evolutionary relationships among hippo and Cetacea (whales, dolphins).[11]
Until 1909, naturalists grouped hippos with pigs, based on molar patterns. Several lines of evidence, first from blood proteins, then from molecular systematics[12] and DNA[13][14] and the fossil record, show that their closest living relatives are cetaceans (whales, dolphins, and porpoises).[15] The common ancestor of hippos and whales branched off from Ruminantia and the rest of the even-toed ungulates; the cetacean and hippo lineages split soon afterwards.[13][16]
Artiodactyla
Tylopoda
Artiofabula
Suina
Cetruminantia
Ruminantia
Whippomorpha
Hippopotamidae
Cetacea
Anthracotherium magnum from the Oligocene of Europe
The most recent theory of the origins of Hippopotamidae suggests that hippos and whales shared a common semiaquatic ancestor that branched off from other artiodactyls around 60 million years ago.[13][15] This hypothesised ancestral group likely split into two branches around 54 million years ago.[12]
One branch would evolve into cetaceans, possibly beginning about 52 million years ago, with the protowhale Pakicetus and other early whale ancestors collectively known as Archaeoceti, which eventually underwent aquatic adaptation into the completely aquatic cetaceans.[16] The other branch became the anthracotheres, a large family of four-legged beasts, the earliest of which in the late Eocene would have resembled skinny hippos with comparatively small and narrow heads. All branches of the anthracotheres, except that which evolved into Hippopotamidae
