Sister group

A sister group or sister taxon is a phylogenetic term denoting the closest relatives of another given unit in an evolutionary tree.[1] The expression is most easily illustrated by a cladogram:


  Taxon A

  Taxon B


  Taxon C

The sister group to A is B; conversely, the sister group to B is A. Groups A and B, together with all other descendants of their most recent common ancestor, form the clade AB. The sister group to clade AB is C.

The whole clade ABC is itself a subtree of a larger tree, which offers yet more sister group branches that are related but farther removed from the leaf nodes, such as A, B, and C.

In cladistic standards, A, B, and C may represent specimens, species, taxon-groups, etc. If they represent species, the term sister species is sometimes used.

The term "sister group" is used in phylogenetic analysis, and only groups identified in the analysis are labeled as sister groups. An example is in birds, whose sister group is commonly cited as the crocodiles, but that is true only when dealing with extant taxa.[2][3] The bird family tree is rooted in the dinosaurs, making for a number of extinct groups branching off before coming to the last common ancestor of birds and crocodiles.[4] Thus, the term sister group must be seen as a relative term, with the caveat that the sister group is the closest relative only among the groups/species/specimens that are included in the analysis.[5]


  1. ^ Eernisse, D.J. "Introduction to Phylogeny: What is a Sister Taxon?". Biology 404 - Evolution. Department of Biological Science, California State University, Fullerton. Retrieved 4 January 2012.
  2. ^ Padian, Kevin; Lindberg, David R.; Polly, Paul David (1 May 1994). "Cladistics and the Fossil Record: The Uses of History". Annual Review of Earth and Planetary Sciences. 22 (1): 63–89. doi:10.1146/annurev.ea.22.050194.000431.
  3. ^ Kemp, T.S. (1 January 1988). "Haemothermia or Archosauria? The interrelationships of mammals, birds and crocodiles". Zoological Journal of the Linnean Society. 92 (1): 67–104. doi:10.1111/j.1096-3642.1988.tb01527.x.
  4. ^ Hughes, J.M. "Ancient bird-crocdilian ancestor uncovered". AVES VITAE - The lives of birds. Archived from the original on 26 April 2012. Retrieved 4 January 2012.
  5. ^ Podani, J. (2010). "Taxonomy in Evolutionary Perspective - An essay on the relationships between taxonomy and evolutionary theory". Synbiologia Hungarica. 5: 1–42.

Amphiesmenoptera is an insect superorder, established by S. G. Kiriakoff, but often credited to Willi Hennig in his revision of insect taxonomy for two sister orders: Lepidoptera (butterflies and moths) and Trichoptera (caddisflies). In 2017, a third fossil order was added to the group, the Tarachoptera.Trichoptera and Lepidoptera share a number of derived characters (synapomorphies) which demonstrate their common descent:

Females, rather than males, are heterogametic (i.e. their sex chromosomes differ).

Dense setae are present in the wings (modified into scales in Lepidoptera).

There is a particular venation pattern on the forewings (the double-looped anal veins).

Larvae have mouth structures and glands to make and manipulate silk.Thus these two extant orders are sisters, with Tarachoptera basal to both groups. Amphiesmenoptera probably evolved in the Jurassic. Lepidoptera differ from the Trichoptera in several features, including wing venation, form of the scales on the wings, loss of the cerci, loss of an ocellus, and changes to the legs.Amphiesmenoptera are thought to be the sister group of Antliophora, a proposed superorder comprising Diptera (flies), Siphonaptera (fleas) and Mecoptera (scorpionflies). Together, Amphiesmenoptera and Antliophora compose the group Mecopterida.


Arachnomorpha is a subdivision or clade of Arthropoda, comprising the monophyletic group formed by the trilobites, other great appendage arthropods and trilobite-like families (Helmetiidae, Xandarellidae, Naraoiidae, Liwiidae, and Tegopeltidae), and a diverse sister clade including the chelicerates. Great debate is held on the position of the Pycnogonida, which are currently thought not to be placed in the immediate vicinity of the Chelicerata. Arachnomorpha are considered the sister group to the crustaceans, which are increasingly being accepted as members of the mandibulate clade (including insects and myriapods).The arachnomorph concept has been challenged by suggestions that the trilobites fall in the mandibulata stem-group.There is no consensus as to assigning Arachnomorpha a formal Linnean rank.

A proposal, which contraposed many synapomorphies uniting them to the Trilobita instead. Consideration of the Olenellinae as sister group to the Chelicerata has been refuted.


Australaves is a recently defined clade of birds, consisting of the Eufalconimorphae (passerines, parrots and falcons) as well as the Cariamiformes (including seriemas and the extinct "terror birds"). They appear to be the sister group of Afroaves. As in the case of Afroaves, the most basal clades have predatory extant members, suggesting this was the ancestral lifestyle; however, some researchers like Darren Naish are skeptical of this assessment, since some extinct representatives such as the herbivorous Strigogyps lead other lifestyles. Basal parrots and falcons are at any rate vaguely crow-like and probably omnivorous.

Cladogram of Telluraves relationships based on Prum, R.O. et al. (2015).

Basal (phylogenetics)

In phylogenetics, basal is the direction of the base (or root) of a rooted phylogenetic tree or cladogram. The term may be more strictly applied only to nodes adjacent to the root, or more loosely applied to nodes regarded as being close to the root. Each node in the tree corresponds to a clade; i.e., clade C may be described as basal within a larger clade D if its root is directly linked to the root of D. The terms deep-branching or early-branching are similar in meaning.

While there must always be two or more equally basal clades sprouting from the root of every cladogram, those clades may differ widely in taxonomic rank and/or species diversity. If C is a basal clade within D that has the lowest rank of all basal clades within D, C may be described as the basal taxon of that rank within D. Greater diversification may be associated with more evolutionary innovation, but ancestral characters should not be imputed to the members of a less species-rich basal clade without additional evidence, as there can be no assurance such an assumption is valid.In general, clade A is more basal than clade B if B is a subgroup of the sister group of A. Within large groups, "basal" may be used loosely to mean 'closer to the root than the great majority of', and in this context terminology such as "very basal" may arise. A 'core clade' is a clade representing all but the basal clade(s) of lowest rank within a larger clade; e.g., core eudicots.


The Euarchonta are a proposed grandorder of mammals containing four orders: the Scandentia or treeshrews, the Dermoptera or colugos, the extinct Plesiadapiformes, and the Primates.

The term "Euarchonta" (Waddell et al. 1999, meaning "true ancestors") appeared in 1999, when molecular evidence suggested that the morphology-based Archonta should be trimmed down to exclude Chiroptera (Waddell et al. 1999b). Further DNA sequence analyses (Madsen et al. 2001, Murphy et al., 2001 Waddell et al. 2001) supported the Euarchonta hypothesis. Despite multiple papers pointing out that some mitochondrial sequences showed unusual properties (particularly murid rodents and hedgehogs) and were likely distorting the overall tree (Sullivan and Swofford 1997, Waddell et al. 1999c), and despite Waddell et al. (2001) showing near total congruence of mtDNA-based and nuclear-based trees when such sequences were excluded, some authors continued to produce misleading trees (Arnason et al., 2002). A study investigating retrotransposon presence/absence data has claimed strong support for Euarchonta (Kriegs et al., 2007). Some interpretations of the molecular data link Primates and Dermoptera in a clade (mirorder) known as Primatomorpha, which is the sister of Scandentia. In some, the Dermoptera are a member of the primates rather than a sister group. Other interpretations link the Dermoptera and Scandentia together in a group called Sundatheria as the sister group of the primates.

Euarchonta and Glires together form the Euarchontoglires, one of the four Eutherian clades.

The current hypothesis, based on molecular clock evidence, suggests that the Euarchonta arose in the Cretaceous period, about 88 million years ago, and diverged 86.2 million years ago into the groups of tree shrews and Primatomorpha. The latter diverged prior to 79.6 million years into the orders of Primates and Dermoptera. The earliest fossil species often ascribed to Euarchonta (Purgatorius coracis) dates to the early Paleocene, 65 million years ago, but it appears to have been a non-placental eutherian. Although it is known that Scandentia is one of the most basal Euarchontoglire clades, the exact phylogenetic position is not yet considered resolved, and it may be a sister of Glires, Primatomorpha or Dermoptera or to all other Euarchontoglires.


Eulipotyphla ("truly fat and blind") is an order of mammals suggested by molecular methods of phylogenetic reconstruction, and includes the laurasiatherian members of the now-invalid polyphyletic order Lipotyphla, but not the afrotherian members (tenrecs and golden moles, now in their own order Afrosoricida). Lipotyphla in turn had been derived by removing a number of groups from Insectivora, the previously used wastebasket taxon.

Eulipotyphla comprises the hedgehogs and gymnures (family Erinaceidae, formerly also the order Erinaceomorpha), solenodons (family Solenodontidae), the desmans, moles, and shrew-like moles (family Talpidae) and true shrews (family Soricidae). True shrews, talpids and solenodons were formerly grouped in Soricomorpha; however, Soricomorpha has been found to be paraphyletic, since erinaceids are the sister group of shrews.


Eumetazoa (Greek: εὖ [eu], well + μετά [metá], after + ζῷον [zóon], animal) or Diploblasts, or Epitheliozoa, or Histozoa are a proposed basal animal clade as a sister group of the Porifera. The basal Eumetazoan clades are the Ctenophora and the ParaHoxozoa. Placozoa is now also seen as a Eumetazoan in the Parahoxozoa.

Several other extinct or obscure life forms, such as Iotuba and Thectardis appear to have emerged in the group. Characteristics of eumetazoans include true tissues organized into germ layers, the presence of neurons, and an embryo that goes through a gastrula stage.

Some phylogenists have speculated the sponges and eumetazoans evolved separately from single-celled organisms, which would mean that the animal kingdom does not form a clade (a complete grouping of all organisms descended from a common ancestor). However, genetic studies and some morphological characteristics, like the common presence of choanocytes, support a common origin.Traditionally, Eumetazoans are a major group of animals in the Five Kingdoms classification of Lynn Margulis and K. V. Schwartz, comprising the Radiata and Bilateria — all animals except the sponges. When treated as a formal taxon Eumetazoa is typically ranked as a subkingdom. The name Metazoa has also been used to refer to this group, but more often refers to the Animalia as a whole. Many classification schemes do not include a subkingdom Eumetazoa.


Exafroplacentalia or Notolegia is a clade of placental mammals proposed in 2001 on the basis of molecular research.Exafroplacentalia places Xenarthra as a sister group to the Boreoeutheria (comprising Laurasiatheria and Euarchontoglires), thus making Afrotheria a primitive group of placental mammals (the group name roughly means "those which are not African placentals").


Ferungulata or Fereuungulata is a clade of placental mammals that groups together various carnivorans and ungulates. It has existed in two guises, a traditional one based on morphological analysis and a revised one taking into account more recent molecular analyses.

The traditional Ferungulata was established by George Gaylord Simpson in 1945. It grouped together the extant orders Carnivora, Perissodactyla and Artiodactyla with the Tubulidentata and the superorder Paenungulata, as well as a number of orders known only from fossils. Although Simpson placed whales (Cetacea) in a separate cohort, recent evidence linking them to Artiodactyla would mean that they belong here as well.

Simpson established the grouping on the basis of morphological criteria, but this traditional understanding of Ferungulata has been challenged by a more recent classification, relying upon genetic criteria. These studies separated his ungulate orders into two distinct placental groups, within Afrotheria and Laurasiatheria, respectively. The 'true' ungulates, Perissodactyla and Artiodactyla, along with the whales, are included in the revised group, along with the Carnivora, and with the addition of pangolins (Pholidota), but the Tubulidentata and paenungulates are excluded. To reflect this difference, the revised clade is usually referred to as Fereuungulata. The Fereuungulata is a sister group to the Chiroptera (bats) and together they make up Scrotifera.


Gnathostomata are the jawed vertebrates. The term derives from Greek: γνάθος (gnathos) "jaw" + στόμα (stoma) "mouth". Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all living vertebrates. In addition to opposing jaws, living gnathostomes have teeth, paired appendages, and a horizontal semicircular canal of the inner ear, along with physiological and cellular anatomical characters such as the myelin sheathes of neurons. Another is an adaptive immune system that uses V(D)J recombination to create antigen recognition sites, rather than using genetic recombination in the variable lymphocyte receptor gene.It is now assumed that Gnathostomata evolved from ancestors that already possessed a pair of both pectoral and pelvic fins. Until recently these ancestors, known as antiarchs, were thought to have lacked pectoral or pelvic fins. In addition to this, some placoderms were shown to have a third pair of paired appendages, that had been modified to claspers in males and basal plates in females--a pattern not seen in any other vertebrate group.The Osteostraci are generally considered the sister taxon of Gnathostomata.It is believed that the jaws evolved from anterior gill support arches that had acquired a new role, being modified to pump water over the gills by opening and closing the mouth more effectively – the buccal pump mechanism. The mouth could then grow bigger and wider, making it possible to capture larger prey. This close and open mechanism would, with time, become stronger and tougher, being transformed into real jaws.

Newer research suggests that a branch of Placoderms was most likely the ancestor of present-day gnathostomes. A 419-million-year-old fossil of a placoderm named Entelognathus had a bony skeleton and anatomical details associated with cartilaginous and bony fish, demonstrating that the absence of a bony skeleton in Chondrichthyes is a derived trait. The fossil findings of primitive bony fishes such as Guiyu oneiros and Psarolepis, which lived contemporaneously with Entelognathus and had pelvic girdles more in common with placoderms than with other bony fish, show that it was a relative rather than a direct ancestor of the extant gnathostomes. It also indicates that spiny sharks and Chondrichthyes represent a single sister group to the bony fishes. Fossils findings of juvenile placoderms, which had true teeth that grew on the surface of the jawbone and had no roots, making it impossible to replace or regrow as they broke or wore down as they grew older, proves the common ancestor of all gnathostomes had teeth and place the origin of teeth along with, or soon after, the evolution of jaws.Late Ordovician-aged microfossils of what have been identified as scales of either acanthodians or "shark-like fishes", may mark Gnathostomata's first appearance in the fossil record. Undeniably unambiguous gnathostome fossils, mostly of primitive acanthodians, begin appearing by the early Silurian, and become abundant by the start of the Devonian.


Heteroneura is a natural group (or clade) in the insect order Lepidoptera that comprises over 99% of all butterflies and moths. This is the sister group of the infraorder Exoporia (swift moths and their relatives), and is characterised by wing venation which is not similar or homoneurous in both pairs of wings.


Iguania is an infraorder of squamate reptiles that includes iguanas, chameleons, agamids, and New World lizards like anoles and phrynosomatids. Using morphological features as a guide to evolutionary relationships, the Iguania are believed to form the sister group to the remainder of the Squamata. However, molecular information has placed Iguania well within the Squamata as sister taxa to the Anguimorpha and closely related to snakes. Iguanians are largely arboreal and usually have primitive fleshy, non-prehensile tongues, although the tongue is highly modified in chameleons. The group has a fossil record that extends back to the Early Jurassic (the oldest known member is Bharatagama, which lived about 190 million years ago in what is now India).


Omalisidae are a very small family of beetles within the superfamily Elateroidea. Members of this beetle family have bioluminescent organs on the larvae. The most recent evidence indicates they are the sister group to a clade comprising the families Rhagophthalmidae and Phengodidae (glowworm beetles).


The ParaHoxozoa are a proposed basal Diploblast/Eumetazoa clade as sister of the Ctenophora. It consists of the Triploblasts/Bilateria as well as the Placozoa and Cnidaria.


The Pooideae are the largest subfamily of the grass family Poaceae, with over 4,200 species in 14 tribes and roughly 200 genera. They include some major cereals such as wheat, barley, oat, rye and many lawn and pasture grasses. They are often referred to as cool-season grasses, because they are distributed in temperate climates. All of them use the C3 photosynthetic pathway.

The Pooideae are the sister group of the bamboos within the BOP clade, and are themselves subdivided into 14 tribes.


The Primatomorpha are a mirorder of mammals containing two extant orders: the Dermoptera or colugos and the Primates (Plesiadapiformes, Strepsirrhini, Haplorhini).

The term "Primatomorpha" first appeared in the general scientific literature in 1991 (K.C. Beard) and 1992 (Kalandadze, Rautian). Major DNA sequence analyses of predominantly nuclear sequences (Murphy et al., 2001) support the Euarchonta hypothesis, while a major study investigating mitochondrial sequences supports a different tree topology (Arnason et al., 2002). A study investigating retrotransposon presence/absence data has claimed strong support for Euarchonta (Kriegs et al., 2007). Some interpretations of the molecular data link Primates and Dermoptera in a clade (mirorder) known as Primatomorpha, which is the sister of Scandentia.

Primates probably split from the Dermoptera sister group 79.6 million years ago during the Cretaceous.Other interpretations link the Dermoptera and Scandentia together in a group called Sundatheria as the sister group of the primates. Some recent studies place Scandentia as sister of the Glires, invalidating Euarchonta.


Scrotifera is a clade of placental mammals that comprises the following orders and their common ancestors: Chiroptera, Carnivora, Pholidota, Perissodactyla and Cetartiodactyla, with the latter including the traditional orders Artiodactyla and Cetacea. Scrotifera is the sister group to the Eulipotyphla and together they make up the Laurasiatheria.


Telluraves (also called land birds or core landbirds) is a recently defined clade of birds with controversial content. Based on most recent genetic studies, the clade unites a variety of bird groups, including the australavians (passerines, parrots, seriamas, and falcons) as well as the afroavians (including the Accipitrimorphae – eagles, hawks, buzzards, vultures etc. – owls and woodpeckers, among others). They appear to be the sister group of a newly defined clade centered on Aequornithes.Given that the most basal extant members of both Afroaves (Accipitrimorphae, Strigiformes) and Australaves (Cariamiformes, Falconiformes) are carnivorous, it has been suggested that the last common ancestor of all Telluraves was probably a predator. Other researchers are skeptical of this assessment, citing the herbivorous cariamiform Strigogyps as evidence to the contrary.

Cladogram of Telluraves relationships based on Prum, R.O. et al. (2015) with some clade names after Yury, T. et al. (2013) and Kimball et al. 2013.

Relevant fields
Basic concepts
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Current topics
Group traits
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