Mammalian Phylogeny


Living mammals can be divided into three subclasses (monotremes, marsupials and placentals) and within these, about 27 orders. Final resolution of the relationships between the orders is only now being achieved with the increased availability of deoxyribonucleic acid (DNA) sequences. Highlights include the deep division of placental mammals into African (Afrotheria), South American (Xenarthra) and northern hemisphere (Boreoeutheria) super‐orders, and the finding that the once considered primitive ‘Insectivora’ and ‘Edentata’ clades, in fact, have members distributed widely among these super‐orders. Another surprise finding from DNA studies has been that whale origins lie among the even‐toed ungulates (Artiodactyla). Our order, Primates is most closely related to the flying lemurs and next, the tree shrews. With the mammal phylogeny becoming well resolved, it is increasingly being used as a framework for inferring evolutionary and ecological processes, such as adaptive radiation.

Key Concepts:

  • A phylogeny describes the relationships of organisms with respect to their shared ancestry.

  • In the context of phylogenetics, molecular data refer to the genotype (DNA) and its direct translations (e.g. protein sequences).

  • Morphology refers to an organism's structural phenotype, sometimes limited to anatomy, but often also incorporating physiology.

  • Homoplasy describes character states that are similar for reasons other than shared ancestry, such as due to convergent or parallel evolution.

  • Retroposons are DNA sequences that can use the protein, reverse transcriptase to copy themselves and insert into new positions in the genome.

  • Molecular dating employs DNA or protein sequence differences between organisms to provide evolutionary time scales, with independent (especially fossil) information used to convert molecular differences into time.

  • Adaptive radiation describes the (often) rapid diversification of closely related species into a wide variety of ecological niches.

Keywords: DNA; biogeography; mammal phylogeny; marsupials; monotremes; morphology; placentals; primates; whales

Figure 1.

Overview of the evolutionary relationships of mammals. Lineage colours indicate the ecological niches of the main lineages. Relationships are based mainly on Waddell et al., Murphy et al., Phillips et al. and Nishihara et al.. The ancestral mammalian niche of small insectivores is shown as a dotted line, and other ecological/morphological transitions are derived from this. The colour of the names indicates inferred geographical origins. The Cretaceous‐Tertiary (Paleogene) boundary (65.5 Mya) is dashed in red. An alternative placement within Laurasiatheria is indicated with a curved arrow.

Figure 2.

Crown (and Total [=stem+crown]) group nomenclature and approximate numbers of species for the three living mammal groups: Monotremata (Australosphenida) (this image is covered by the Creative Commons Attribition‐Share Alike License), Marsupialia (Metatheria) and Placentalia (Eutheria). Molecular clock evolutionary time 95% confidence intervals in millions of years (from Phillips et al., ) are placed at each divergence. Species examples are short‐beaked echidna (Tachglossus aculeatus), red kangaroo (Macropus rufus) and donkey (Equus africanus).



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Further Reading

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Phillips, Matthew J, and Penny, David(Sep 2010) Mammalian Phylogeny. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0005306.pub2]