Cetartiodactyla - Kin To Hippos

Kin To Hippos

The idea that whales evolved from within the Artiodactyla is based on analysis of DNA sequences. In the initial molecular analyses, whales were shown to be more closely related to ruminants (such as cattle and deer) than ruminants are to pigs. For the order name to reflect a real evolutionary unit, the term Cetartiodactyla was coined.

Later molecular analyses included a wider sampling of artiodactyls and produced a more complete tale. Hippos were determined to be the closest relative of whales, ruminants were related to a whale/hippo clade, and pigs were more distant. In addition to producing the controversial whale/hippo clade, these analyses debunked the idea that hippos and pigs are closely related. This had been a popular taxonomic hypothesis (Suiformes) based on similarities in morphological (physical) characteristics.

In addition to DNA and protein sequences, researchers tracked the movement of transposons called SINEs in the genome (see the method at retrotransposon marker). A transposon is a DNA sequence that will occasionally make a copy of itself and insert that copy into another part of the genome. It is considered highly unlikely that SINEs will insert themselves into exactly the same part of a genome by chance. The data indicate several transposons inserted themselves at the same point in the genomes of whales, ruminants and hippos (sometimes referred to as "pseudoruminants" because, although they have four-chambered stomachs like true ruminants, they do not chew the cud). This insertion point is not shared with camels and pigs.

This hypothesis has been tested with DNA sequences from a host of genes: the complete mitochondrial genome (as well as several of its genes independently), beta-casein, kappa-casein, von Willebrand factor, breast cancer 1, recombination activating genes 1 and 2, cannabinoid receptor 1, and several others. These sequence data and the transposons converge on the same conclusion that hippos and whales are more closely related to one another than either is to other artiodactyls.

Sequences analyzed in combined analyses with morphological characters have also produced the same results as sequences alone. Some have argued the sheer number of characters (one for each nucleotide) in sequences swamps out the effects of morphology. A few morphology-based studies have suggested (weakly) the same results as the molecular results, but, overall, most morphological studies have conflicted with the whale/hippo hypothesis of Cetartiodactyla.

An important exception is a recent study conducted by Boisserie et al. (2005). They examined 80 hard morphological characters of fossil and extant cetartiodactylan taxa. Their results suggest the hippopotamids evolved from within a clade of anthracotheres. That anthracothere/hippopotamid clade appears to be sister to the Cetacea and supports the molecular results.