Basal (phylogenetics)

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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. Note that extant taxa that lie on branches connecting directly to the root are not more closely related to the root than any other extant taxa. [1] [2] [3]

Contents

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, [n 1] species diversity, or both. [n 2] If C is a basal clade within D that has the lowest rank of all basal clades within D, [n 3] C may be described as the basal taxon of that rank within D. [n 4] The concept of a 'key innovation' implies some degree of correlation between evolutionary innovation and diversification. [4] [5] [6] [n 5] However, such a correlation does not make a given case predicable, so ancestral characters should not be imputed to the members of a less species-rich basal clade without additional evidence. [1] [2] [7] [8] [n 6]

In general, clade A is more basal than clade B if B is a subgroup of the sister group of A or of A itself. [n 7] In the context of large groups, the term "basal" is often used loosely to refer to positions closer to the root than the majority, and in such cases, you might encounter expressions like "very basal." A 'core clade' refers to the grouping that encompasses all constituent clades except for the basal clade(s) of the lowest rank within a larger clade, exemplified by core eudicots. No extant taxon is closer to the root than any other.

Usage

A basal group in the stricter sense forms a sister group to the rest of the larger clade,[ citation needed ] as in the following case:

Root  

 Basal clade #1

 Non-basal clade #1

 Non-basal clade #2  

 Non-basal clade #3

Basal clade #2

While it is easy to identify a basal clade in such a cladogram, the appropriateness of such an identification is dependent on the accuracy and completeness of the diagram. It is often assumed in this example that the terminal branches of the cladogram depict all the extant taxa of a given rank within the clade; this is one reason the term basal is highly deceptive, as the lack of additional species in one clade is taken as evidence of morphological affinity with ancestral taxa. Additionally, this qualification does not ensure that the diversity of extinct taxa (which may be poorly known) is represented.[ citation needed ]

In phylogenetics, the term basal cannot be objectively applied to clades of organisms, but tends to be applied selectively and more controversially to groups or lineages [n 8] thought to possess ancestral characters, or to such presumed ancestral traits themselves. In describing characters, "ancestral" or "plesiomorphic" are preferred to "basal" or "primitive", the latter of which may carry false connotations of inferiority or a lack of complexity. [1] The terms ''deep-branching'' or ''early-branching'' are similar in meaning, and equally may misrepresent extant taxa that lie on branches connecting directly to the root node as having more ancestral character states. [1] [2]

Despite the ubiquity of the usage of basal, systematists try to avoid its usage because its application to extant groups is unnecessary and misleading. [3] The term is more often applied when one branch (the one deemed "basal") is less diverse than another branch (this being the situation in which one would expect to find a basal taxon of lower minimum rank). The term may be equivocal in that it also refers to the direction of the root of the tree, which represents a hypothetical ancestor; this consequently may inaccurately imply that the sister group of a more species-rich clade displays ancestral features. [8] An extant basal group may or may not resemble the last common ancestor of a larger clade to a greater degree than other groups, and is separated from that ancestor by the same amount of time as all other extant groups. However, there are cases where the unusually small size of a sister group does indeed correlate with an unusual number of ancestral traits, as in Amborella (see below). This is likely a source of the mis-use of the term. Other famous examples of this phenomenon are the oviparous reproduction and nipple-less lactation of monotremes, a clade of mammals [11] with just five species, and the archaic anatomy of the tuatara, [12] a basal clade of lepidosaurian with a single species.

Examples

Flowering plants

Amborella trichopoda, the most basal extant angiosperm Amborella2.jpg
Amborella trichopoda , the most basal extant angiosperm

The flowering plant family Amborellaceae, restricted to New Caledonia in the southwestern Pacific, [n 9] is a basal clade of extant angiosperms, [13] consisting of the most species, genus, family and order within the group that are sister to all other angiosperms (out of a total of about 250,000 angiosperm species). The traits of Amborella trichopoda are regarded as providing significant insight into the evolution of flowering plants; for example, it has "the most primitive wood (consisting only of tracheids), of any living angiosperm" as well as "simple, separate flower parts of indefinite numbers, and unsealed carpels". [14] However, those traits are a mix of archaic and apomorphic (derived) features that have only been sorted out via comparison with other angiosperms and their positions within the phylogenetic tree (the fossil record could potentially also be helpful in this respect, but is absent in this case). [14] The cladogram below is based on Ramírez-Barahona et al. (2020), [15] with species counts taken from the source indicated.

  Angiosperms  

Amborellales (1 species) [16]

Nymphaeales (about 90 species) [17]

Austrobaileyales (about 95 species) [17]

Magnoliids (about 9,000 species) [16]

Chloranthales (about 80 species) [17]

Monocots (about 70,000 species) [16]

Ceratophyllales (about 6 species) [16]

Eudicots (about 175,000 species) [16]

Great apes

Within the great apes, gorillas (eastern and western) are a sister group to chimpanzees, bonobos and humans. These five species form a clade, the subfamily Homininae (African apes), of which Gorilla has been termed the basal genus. However, if the analysis is not restricted to genera, the Homo plus Pan clade is also basal.

The phylogenetic tree of marsupials derived from retroposon data shows the basal position of South American Didelphimorphia within Marsupialia, and the basal position of South American Dromiciops within otherwise Australasian Australidelphia. Phylogenetic tree of marsupials derived from retroposon data - journal.pbio.1000436.g002.png
The phylogenetic tree of marsupials derived from retroposon data shows the basal position of South American Didelphimorphia within Marsupialia, and the basal position of South American Dromiciops within otherwise Australasian Australidelphia.
Relationship of biogeography and phylogeny of bat superfamily Noctilionoidea inferred from nuclear DNA sequence data, showing the basal position of the Malagasy family Myzopodidae. Locations with only fossil members are indicated by red stars. Noctilionoidea phylogeny PLoS ONE 2014-02-04.png
Relationship of biogeography and phylogeny of bat superfamily Noctilionoidea inferred from nuclear DNA sequence data, showing the basal position of the Malagasy family Myzopodidae. Locations with only fossil members are indicated by red stars.
  Homininae  

Humans (Homo sapiens)

Bonobos (Pan paniscus)

Chimpanzees (Pan troglodytes)

Eastern gorillas (Gorilla beringei)

Western gorillas (Gorilla gorilla)

Moreover, orangutans are a sister group to Homininae and are the basal genus in the great ape family Hominidae as a whole.

  Hominidae  
  Ponginae  

Orangutans (Pongo spp.)

 Homininae 

Humans (Homo sapiens)

Chimpanzees (Pan spp.)

Gorillas (Gorilla spp.)

Subfamilies Homininae and Ponginae are both basal within Hominidae, but given that there are no nonbasal subfamilies in the cladogram it is unlikely the term would be applied to either. In general, a statement to the effect that one group (e.g., orangutans) is basal, or branches off first, within another group (e.g., Hominidae) may not make sense unless the appropriate taxonomic level(s) (genus, in this case) is specified. If that level cannot be specified (i.e., if the clade in question is unranked) a more detailed description of the relevant sister groups may be needed. As can be seen, the term is not reflective of ancestral states or proximity to the common ancestor of extant species.

In this example, orangutans differ from the other genera in their Asian range. This fact plus their basal status provides a hint that the most recent common ancestor of extant great apes may have been Eurasian (see below), a suggestion that is consistent with other evidence. [19] (Of course, lesser apes are entirely Asiatic.) However, orangutans also differ from African apes in their more highly arboreal lifestyle, a trait generally viewed as ancestral among the apes. [20] [21]

Relevance to biogeographic history

Given that the deepest phylogenetic split in a group is likely to have occurred early in its history, identification of the most basal subclade(s) in a widely dispersed taxon or clade can provide valuable insight into its region of origin; however, the lack of additional species in a clade is not evidence that it carries the ancestral state for most traits. Most deceptively, people often believe that the direction of migration away from the area of origin can also be inferred (as in the Amaurobioides and Noctilionoidea cases below). As with all other traits, the phylogeographic location of one clade that connects to the root does not provide information about the ancestral state. Examples where such unjustified inferences may have been made include:

Notes

  1. Meaning the lowest taxonomic ranks of the respective clades; the highest ranks should be the same (assuming they are ranked).
  2. See the Amborella example, in which one basal clade is a single extant species (that is also the sole living representative of an order, Amborellales). Meanwhile, the other (unranked) sister basal clade has about 250,000 species.
  3. For example, C might be a genus and the other basal clade(s) might have the higher ranks of subfamily or family.
  4. In the great apes example, Gorilla is the basal genus of subfamily Homininae, while Pongo is the basal genus of family Hominidae. The two basal clades of the latter both have the highest rank of subfamily, i.e. Homininae and Ponginae.
  5. Greater diversification of a clade may also be associated with colonization of a new land mass, especially if larger or less competitive than the ancestral land mass; see the coral snake, marsupial and noctilionoid bat examples.
  6. For example, the giant panda represents the most basal extant species, genus and subfamily within Ursidae, [9] but its specializations for a bamboo diet are not ancestral ursid characters. [10]
  7. That is, in the diagram below, both basal clades #1 and #2 are more basal than non-basal clade #1, which in turn is more basal than non-basal clades #2 and #3.
  8. Since a lineage is a linear chain of descent, all lineages within a clade can be traced back not only to the root, but to the origin of life. Thus, from a phylogenetic standpoint, the notion of a lineage being basal is nonsensical. However, in genetics, basal lineage refers to a lineage connecting a common ancestor with a single variant allele to a branch ancestor with two descendant variants.
  9. New Caledonia is viewed as a refugium; i.e., in this case the geographic location of the basal clade is not thought to provide evidence for the locale in which angiosperms originated.
  10. These conclusions have been supported by the finding of Eocene fossil remains of the microbiotherian Woodburnodon casei in Antarctica, [30] which is presumed to have served as a way station on the migration route to Australia before the final breakup of Gondwana.
  11. Similarly, among australobatrachian frogs, the South American family Calyptocephalellidae, with 5 extant species (living in the same Valdivian forest as the monito del monte), is basal to the Australasian families Limnodynastidae and Myobatrachidae, [31] with about 120 extant species, suggesting a South American origin for the group. [32] This is consistent with the finding of a fossil from the South American family in Antarctica. [33]
  12. Ratites may have similarly traveled overland from South America to colonize Australia; [34] a fossil ratite is known from Antarctica, [35] and South American rheas are more basal within the group than Australo-Pacific ratites. [34]

Related Research Articles

<span class="mw-page-title-main">Asterales</span> Large order of flowering plants

Asterales is an order of dicotyledonous flowering plants that includes the large family Asteraceae known for composite flowers made of florets, and ten families related to the Asteraceae. While asterids in general are characterized by fused petals, composite flowers consisting of many florets create the false appearance of separate petals.

Cladistics is an approach to biological classification in which organisms are categorized in groups ("clades") based on hypotheses of most recent common ancestry. The evidence for hypothesized relationships is typically shared derived characteristics (synapomorphies) that are not present in more distant groups and ancestors. However, from an empirical perspective, common ancestors are inferences based on a cladistic hypothesis of relationships of taxa whose character states can be observed. Theoretically, a last common ancestor and all its descendants constitute a (minimal) clade. Importantly, all descendants stay in their overarching ancestral clade. For example, if the terms worms or fishes were used within a strict cladistic framework, these terms would include humans. Many of these terms are normally used paraphyletically, outside of cladistics, e.g. as a 'grade', which are fruitless to precisely delineate, especially when including extinct species. Radiation results in the generation of new subclades by bifurcation, but in practice sexual hybridization may blur very closely related groupings.

<span class="mw-page-title-main">Clade</span> Group of a common ancestor and all descendants

In biological phylogenetics, a clade, also known as a monophyletic group or natural group, is a grouping of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. In the taxonomical literature, sometimes the Latin form cladus is used rather than the English form. Clades are the fundamental unit of cladistics, a modern approach to taxonomy adopted by most biological fields.

<span class="mw-page-title-main">Homininae</span> Subfamily of mammals

Homininae, also called "African hominids" or "African apes", is a subfamily of Hominidae. It includes two tribes, with their extant as well as extinct species: 1) the tribe Hominini ―and 2) the tribe Gorillini (gorillas). Alternatively, the genus Pan is sometimes considered to belong to its own third tribe, Panini. Homininae comprises all hominids that arose after orangutans split from the line of great apes. The Homininae cladogram has three main branches, which lead to gorillas, and to humans and chimpanzees via the tribe Hominini and subtribes Hominina and Panina. There are two living species of Panina and two living species of gorillas, but only one extant human species. Traces of extinct Homo species, including Homo floresiensis have been found with dates as recent as 40,000 years ago. Organisms in this subfamily are described as hominine or hominines.

<span class="mw-page-title-main">Saxifragales</span> Order of Eudicot flowering plants in the Superrosid clade

Saxifragales is an order of angiosperms, or flowering plants, containing 15 botanical families and around 100 genera, with nearly 2,500 species. Of the 15 families, many are small, with eight of them being monotypic. The largest family is the Crassulaceae (stonecrops), a diverse group of mostly succulent plants, with about 35 genera. Saxifragales are found worldwide, primarily in temperate to subtropical zones, rarely being encountered growing wild in the tropics; however, many species are now cultivated throughout the world as knowledge of plant husbandry has improved. They can be found in a wide variety of environments, from deserts to fully aquatic habitats, with species adapted to alpine, forested or fully-aquatic habitats. Many are epiphytic or lithophytic, growing on exposed cliff faces, on trees or on rocks, and not requiring a highly organic or nutrient-dense substrate to thrive.

<span class="mw-page-title-main">Fabaceae</span> Family of legume flowering plants

The Fabaceae or Leguminosae, commonly known as the legume, pea, or bean family, are a large and agriculturally important family of flowering plants. It includes trees, shrubs, and perennial or annual herbaceous plants, which are easily recognized by their fruit (legume) and their compound, stipulate leaves. The family is widely distributed, and is the third-largest land plant family in number of species, behind only the Orchidaceae and Asteraceae, with about 765 genera and nearly 20,000 known species.

<span class="mw-page-title-main">Ratite</span> Polyphyletic group of birds

A ratite is any of a group of mostly flightless birds within the infraclass Palaeognathae. They are mostly large, long-necked, and long-legged, the exception being the kiwi, which is also the only nocturnal extant ratite.

<span class="mw-page-title-main">Afrotheria</span> Clade of mammals containing elephants and elephant shrews

Afrotheria is a superorder of mammals, the living members of which belong to groups that are either currently living in Africa or of African origin: golden moles, elephant shrews, otter shrews, tenrecs, aardvarks, hyraxes, elephants, sea cows, and several extinct clades. Most groups of afrotheres share little or no superficial resemblance, and their similarities have only become known in recent times because of genetics and molecular studies. Many afrothere groups are found mostly or exclusively in Africa, reflecting the fact that Africa was an island continent from the Cretaceous until the early Miocene around 20 million years ago, when Afro-Arabia collided with Eurasia.

<span class="mw-page-title-main">Haplorhini</span> Suborder of primates

Haplorhini, the haplorhines or the "dry-nosed" primates is a suborder of primates containing the tarsiers and the simians, as sister of the Strepsirrhini ("moist-nosed"). The name is sometimes spelled Haplorrhini. The simians include catarrhines, and the platyrrhines.

<span class="mw-page-title-main">Outgroup (cladistics)</span>

In cladistics or phylogenetics, an outgroup is a more distantly related group of organisms that serves as a reference group when determining the evolutionary relationships of the ingroup, the set of organisms under study, and is distinct from sociological outgroups. The outgroup is used as a point of comparison for the ingroup and specifically allows for the phylogeny to be rooted. Because the polarity (direction) of character change can be determined only on a rooted phylogeny, the choice of outgroup is essential for understanding the evolution of traits along a phylogeny.

<i>Pierolapithecus</i> Extinct species of ape from Miocene Europe

Pierolapithecus catalaunicus is an extinct species of primate which lived around 12.5-13 million years ago during the Miocene in what is now Hostalets de Pierola, Catalonia, Spain. Some researchers believe that it is a candidate for common ancestor to the great ape clade, or is at least closer than any previous fossil discovery. Others suggest it being a pongine, or a dryopith. On 16 October 2023, scientists reported the facial reconstruction of the great ape.

<span class="mw-page-title-main">Hominini</span> Tribe of mammals

The Hominini form a taxonomic tribe of the subfamily Homininae ("hominines"). Hominini includes the extant genera Homo (humans) and Pan and in standard usage excludes the genus Gorilla (gorillas).

<span class="mw-page-title-main">Asterids</span> Clade of eudicot angiosperms

In the APG IV system (2016) for the classification of flowering plants, the name asterids denotes a clade. Asterids is the largest group of flowering plants, with more than 80,000 species, about a third of the total flowering plant species. Well-known plants in this clade include the common daisy, forget-me-nots, nightshades, the common sunflower, petunias, yacon, morning glory, lettuce, sweet potato, coffee, lavender, lilac, olive, jasmine, honeysuckle, ash tree, teak, snapdragon, sesame, psyllium, garden sage, table herbs such as mint, basil, and rosemary, and rainforest trees such as Brazil nut.

Phylogenetic nomenclature is a method of nomenclature for taxa in biology that uses phylogenetic definitions for taxon names as explained below. This contrasts with the traditional method, by which taxon names are defined by a type, which can be a specimen or a taxon of lower rank, and a description in words. Phylogenetic nomenclature is regulated currently by the International Code of Phylogenetic Nomenclature (PhyloCode).

<span class="mw-page-title-main">Hominidae</span> Family of primates

The Hominidae, whose members are known as the great apes or hominids, are a taxonomic family of primates that includes eight extant species in four genera: Pongo ; Gorilla ; Pan ; and Homo, of which only modern humans remain.

<span class="mw-page-title-main">Mesangiospermae</span> One of two clades of flowering plants

Mesangiospermae is a clade of flowering plants (angiosperms), informally called "mesangiosperms". They are one of two main groups of angiosperms. It is a name created under the rules of the PhyloCode system of phylogenetic nomenclature. There are about 350,000 species of mesangiosperms. The mesangiosperms contain about 99.95% of the flowering plants, assuming that there are about 175 species not in this group and about 350,000 that are. While such a clade with a similar circumscription exists in the APG III system, it was not given a name.

The chimpanzee–human last common ancestor (CHLCA) is the last common ancestor shared by the extant Homo (human) and Pan genera of Hominini. Estimates of the divergence date vary widely from thirteen to five million years ago.

<span class="mw-page-title-main">Seed plant</span> Clade of seed plants

A seed plant or spermatophyte, also known as a phanerogam or a phaenogam, is any plant that produces seeds. It is a category of embryophyte that includes most of the familiar land plants, including the flowering plants and the gymnosperms, but not ferns, mosses, or algae.

The phylogenetic split of Hominidae into the subfamilies Homininae and Ponginae is dated to the middle Miocene, roughly 18 to 14 million years ago. This split is also referenced as the "orangutan–human last common ancestor" by Jeffrey H. Schwartz, professor of anthropology at the University of Pittsburgh School of Arts and Sciences, and John Grehan, director of science at the Buffalo Museum.

<span class="mw-page-title-main">Pentapetalae</span> Group of eudicots known as core eudicots

In phylogenetic nomenclature, the Pentapetalae are a large group of eudicots that were informally referred to as the "core eudicots" in some papers on angiosperm phylogenetics. They comprise an extremely large and diverse group accounting for about 65% of the species richness of the angiosperms, with wide variability in habit, morphology, chemistry, geographic distribution, and other attributes. Classical systematics, based solely on morphological information, was not able to recognize this group. In fact, the circumscription of the Pentapetalae as a clade is based on strong evidence obtained from DNA molecular analysis data.

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