Systematics and Evolution of the Pig

According to the current classification, pigs belong to order Cetartiodactyla, which includes even-toed ungulates (the former order Artiodactyla), and whales and dolphins, which are representatives of the former order Cetacea (Murphy et al., 2001; Novacek, 2001). Cetartiodactyla diverged from placental mammals approximately 87.2 million years ago (mya) (Murphy and Eizirik, 2009). There are three well-established suborders: (i) Tylopoda - camels and llamas; (ii) Suiformes (also known as Suina) - pigs and peccaries (and formerly hippos); and (iii) Ruminantia. The fourth and more recently created suborder - called Cetancodonta - includes hippos, dolphins and whales (Price et al., 2005; O'Leary and Gatesy, 2008; Huffman, 2009). Phylogenetic analyses of the cytochrome b sequences from 264 of the 290 extant Cetartiodactyla show that the Suiformes and Ruminantia cluster together as a sister clade of Cetancodonta, while the Tylopoda cluster in a separate clade (Agnarsson and May-Collado, 2008). Molecular genetic studies of the Cetartiodactyla have shown good agreement with the basic structure of their phylogeny as supported by morphological data (Novacek, 1992), but the position of hippos and camels remains contentious (Thewissen et al., 2007; O'Leary and Gatesy, 2008; Spaulding et al., 2009).