In
phylogenetics
In biology, phylogenetics (; from Greek language, Greek wikt:φυλή, φυλή/wikt:φῦλον, φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary his ...
, basal is the direction of the ''base'' (or root) of a
rooted phylogenetic tree or
cladogram
A cladogram (from Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an evolutionary tree because it does not show how ancestors are related to d ...
. 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.
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
In biological classification, taxonomic rank is the relative level of a group of organisms (a taxon) in an ancestral or hereditary hierarchy. A common system consists of species, genus, family (biology), family, order (biology), order, class (b ...
,
species diversity
Species diversity is the number of different species that are represented in a given community (a dataset). The effective number of species refers to the number of equally abundant species needed to obtain the same mean proportional species abundan ...
, or both. 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
In biology, a taxon (back-formation from ''taxonomy''; plural taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular nam ...
of that rank within ''D''. The concept of a '
key innovation In evolutionary biology, a key innovation, also known as an adaptive breakthrough or key adaptation, is a novel phenotypic trait that allows subsequent radiation and success of a taxonomic group. Typically they bring new abilities that allows the ta ...
' implies some degree of correlation between evolutionary innovation and
diversification
Diversification may refer to:
Biology and agriculture
* Genetic divergence, emergence of subpopulations that have accumulated independent genetic changes
* Agricultural diversification involves the re-allocation of some of a farm's resources to n ...
.
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.
In general, clade ''A'' is more basal than clade ''B'' if ''B'' is a subgroup of the
sister group
In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree.
Definition
The expression is most easily illustrated by a cladogram:
Taxon A and t ...
of ''A'' or of ''A'' itself. 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 eudicots, Eudicotidae, or eudicotyledons are a clade of flowering plants mainly characterized by having two seed leaves upon germination. The term derives from Dicotyledons.
Traditionally they were called tricolpates or non-magnoliid dicots ...
. Of course, no extant taxon is closer to the root than any other, by definition.
Usage
A basal group in the stricter sense forms a sister group to the rest of the larger clade, as in the following case:
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.
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 thought to possess ancestral characters, or to such presumed ancestral
traits themselves. In describing characters, "ancestral" or "
plesiomorphic
In phylogenetics, a plesiomorphy ("near form") and symplesiomorphy are synonyms for an ancestral character shared by all members of a clade, which does not distinguish the clade from other clades.
Plesiomorphy, symplesiomorphy, apomorphy, and ...
" are preferred to "basal" or "
primitive", the latter of which may carry false connotations of inferiority or a lack of complexity.
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.
Despite the ubiquity of the usage of ''basal'',
systematists
Biological systematics is the study of the diversification of living forms, both past and present, and the relationships among living things through time. Relationships are visualized as evolutionary trees (synonyms: cladograms, phylogenetic tr ...
try to avoid its usage because its application to extant groups is unnecessary and misleading.
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.
An extant basal group may or may not resemble the
last common ancestor
In biology and genetic genealogy, the most recent common ancestor (MRCA), also known as the last common ancestor (LCA) or concestor, of a set of organisms is the most recent individual from which all the organisms of the set are descended. The ...
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
''Amborella'' is a monotypic genus of understory shrubs or small trees endemic to the main island, Grande Terre, of New Caledonia in the southwest Pacific Ocean. The genus is the only member of the family Amborellaceae and the order Amborellale ...
'' (see below). This is likely a source of the mis-use of the term. Other famous examples of this phenomenon are the
oviparous
Oviparous animals are animals that lay their eggs, with little or no other embryonic development within the mother. This is the reproductive method of most fish, amphibians, most reptiles, and all pterosaurs, dinosaurs (including birds), and ...
reproduction and nipple-less
lactation
Lactation describes the secretion of milk from the mammary glands and the period of time that a mother lactates to feed her young. The process naturally occurs with all sexually mature female mammals, although it may predate mammals. The proces ...
of
monotreme
Monotremes () are prototherian mammals of the order Monotremata. They are one of the three groups of living mammals, along with placentals (Eutheria), and marsupials (Metatheria). Monotremes are typified by structural differences in their brain ...
s, a clade of mammals
with just five species, and the archaic anatomy of the
tuatara
Tuatara (''Sphenodon punctatus'') are reptiles endemic to New Zealand. Despite their close resemblance to lizards, they are part of a distinct lineage, the order Rhynchocephalia. The name ''tuatara'' is derived from the Māori language and m ...
,
a basal clade of
lepidosauria
The Lepidosauria (, from Greek meaning ''scaled lizards'') is a subclass or superorder of reptiles, containing the orders Squamata and Rhynchocephalia. Squamata includes snakes, lizards, and amphisbaenians. Squamata contains over 9,000 species, m ...
n with a single species.
Examples
Flowering plants
![Amborella2](https://upload.wikimedia.org/wikipedia/commons/c/cd/Amborella2.jpg)
The
flowering plant
Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek words ('container, vessel') and ('seed'), and refers to those plants th ...
family
Amborellaceae
''Amborella'' is a monotypic genus of understory shrubs or small trees endemic to the main island, Grande Terre, of New Caledonia in the southwest Pacific Ocean. The genus is the only member of the family Amborellaceae and the order Amborellale ...
, restricted to
New Caledonia
)
, anthem = ""
, image_map = New Caledonia on the globe (small islands magnified) (Polynesia centered).svg
, map_alt = Location of New Caledonia
, map_caption = Location of New Caledonia
, mapsize = 290px
, subdivision_type = Sovereign st ...
in the southwestern Pacific, is a
basal clade of extant angiosperms,
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
tracheid
A tracheid is a long and tapered lignified cell in the xylem of vascular plants. It is a type of conductive cell called a tracheary element. Angiosperms use another type of tracheary element, called vessel elements, to transport water through the ...
s), of any living angiosperm" as well as "simple, separate flower parts of indefinite numbers, and unsealed carpels".
However, those traits are a mix of archaic and
apomorphic
In phylogenetics, an apomorphy (or derived trait) is a novel character or character state that has evolved from its ancestral form (or plesiomorphy). A synapomorphy is an apomorphy shared by two or more taxa and is therefore hypothesized to have ...
(derived) features that have only been sorted out via comparison with other angiosperms and their positions within the phylogenetic tree (the
fossil record
A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved in ...
could potentially also be helpful in this respect, but is absent in this case).
The cladogram below is based on Ramírez-Barahona ''et al.'' (2020),
with species counts taken from the source indicated.
Great apes
Within the great apes,
gorilla
Gorillas are herbivorous, predominantly ground-dwelling great apes that inhabit the tropical forests of equatorial Africa. The genus ''Gorilla'' is divided into two species: the eastern gorilla and the western gorilla, and either four or fi ...
s (eastern and western) are a sister group to
chimpanzee
The chimpanzee (''Pan troglodytes''), also known as simply the chimp, is a species of great ape native to the forest and savannah of tropical Africa. It has four confirmed subspecies and a fifth proposed subspecies. When its close relative th ...
s,
bonobo
The bonobo (; ''Pan paniscus''), also historically called the pygmy chimpanzee and less often the dwarf chimpanzee or gracile chimpanzee, is an endangered great ape and one of the two species making up the genus '' Pan,'' the other being the comm ...
s and
human
Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture, ...
s. These five species form a clade, the subfamily
Homininae
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 (with the genus ''Homo'' including modern humans and numerou ...
(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.
![Phylogenetic tree of marsupials derived from retroposon data - journal](https://upload.wikimedia.org/wikipedia/commons/e/e9/Phylogenetic_tree_of_marsupials_derived_from_retroposon_data_-_journal.pbio.1000436.g002.png)
Moreover,
orangutan
Orangutans are great apes native to the rainforests of Indonesia and Malaysia. They are now found only in parts of Borneo and Sumatra, but during the Pleistocene they ranged throughout Southeast Asia and South China. Classified in the genus ...
s are a sister group to Homininae and are the basal genus in the great ape family
Hominidae
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'' (the Bornean, Sumatran and Tapanuli orangutan); ''Gorilla'' (the ea ...
as a whole.
Subfamilies Homininae and
Ponginae
Ponginae , also known as the Asian hominids, is a subfamily in the family Hominidae. Once a diverse lineage of Eurasian apes, the subfamily has only one extant genus, '' Pongo'' (orangutans), which contains three extant species; the Sumatran oran ...
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.
[.] (Of course,
lesser ape
Gibbons () are apes in the family Hylobatidae (). The family historically contained one genus, but now is split into four extant genera and 20 species. Gibbons live in subtropical and tropical rainforest from eastern Bangladesh to Northeast India ...
s are entirely Asiatic.) However, orangutans also differ from African
ape
Apes (collectively Hominoidea ) are a clade of Old World simians native to sub-Saharan Africa and Southeast Asia (though they were more widespread in Africa, most of Asia, and as well as Europe in prehistory), which together with its siste ...
s in their more highly
arboreal
Arboreal locomotion is the Animal locomotion, locomotion of animals in trees. In habitats in which trees are present, animals have evolved to move in them. Some animals may scale trees only occasionally, but others are exclusively arboreal. Th ...
lifestyle, a trait generally viewed as ancestral among the apes.
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:
* Spiders of the genus ''
Amaurobioides
''Amaurobioides'' is a genus of Anyphaenidae, anyphaenid sac spiders first described by Octavius Pickard-Cambridge, O. Pickard-Cambridge in 1883.
Species
it contains twelve species:
*''Amaurobioides africana'' Hewitt, 1917 – Namibia, South Af ...
'' are present in South Africa, Australia, New Zealand and Chile.
The most basal clade is South African; DNA sequence evidence indicates that after their South American ancestors reached South Africa, they
dispersed eastward all the way back to South America over an interval of about 8 million years.
*
Iguanid
The Iguanidae is a family of lizards composed of the iguanas, chuckwallas, and their prehistoric relatives, including the widespread green iguana.
Taxonomy
Iguanidae is thought to be the sister group to the collared lizards (family Crotaphy ...
lizards (''
sensu lato
''Sensu'' is a Latin word meaning "in the sense of". It is used in a number of fields including biology, geology, linguistics, semiotics, and law. Commonly it refers to how strictly or loosely an expression is used in describing any particular co ...
'') are distributed throughout the Americas, on
Madagascar
Madagascar (; mg, Madagasikara, ), officially the Republic of Madagascar ( mg, Repoblikan'i Madagasikara, links=no, ; french: République de Madagascar), is an island country in the Indian Ocean, approximately off the coast of East Africa ...
, and on
Fiji
Fiji ( , ,; fj, Viti, ; Fiji Hindi: फ़िजी, ''Fijī''), officially the Republic of Fiji, is an island country in Melanesia, part of Oceania in the South Pacific Ocean. It lies about north-northeast of New Zealand. Fiji consists ...
and
Tonga
Tonga (, ; ), officially the Kingdom of Tonga ( to, Puleʻanga Fakatuʻi ʻo Tonga), is a Polynesian country and archipelago. The country has 171 islands – of which 45 are inhabited. Its total surface area is about , scattered over in ...
in the western South Pacific. The
Malagasy forms (Opluridae) were previously thought to be basal, with an estimated divergence date from the others of ~162 million years, not long before the time of Madagascar's separation from Africa.
This suggested that iguanids once had a widespread
Gondwanan
Gondwana () was a large landmass, often referred to as a supercontinent, that formed during the late Neoproterozoic (about 550 million years ago) and began to break up during the Jurassic period (about 180 million years ago). The final stages ...
distribution; after the Malagasy and New World representatives were separated by
vicariance
Allopatric speciation () – also referred to as geographic speciation, vicariant speciation, or its earlier name the dumbbell model – is a mode of speciation that occurs when biological populations become geographically isolated from ...
, less isolated Old World iguanids became extinct through competition with other lizard groups (e.g.,
agamid
Agamidae is a family of over 300 species of iguanian lizards indigenous to Africa, Asia, Australia, and a few in Southern Europe. Many species are commonly called dragons or dragon lizards.
Overview
Phylogenetically, they may be sister to the I ...
s). In contrast,
western Pacific iguanids are nested deeply within American iguanids,
having apparently colonized their isolated range after an epic 10,000 km
rafting event
Oceanic dispersal is a type of biological dispersal that occurs when terrestrial organisms transfer from one land mass to another by way of a sea crossing. Island hopping is the crossing of an ocean by a series of shorter journeys between islan ...
.
However, a 2022 study found oplurids to be closely allied with the American iguanians
Leiosauridae
Leiosauridae is a family of iguanian lizards containing six genera and 34 species. The family is endemic to Central America and South America.
Taxonomy
A 2022 phylogenetic study found the Leiosauridae to be the sister taxa to the Opluridae, a ...
, having only diverged 60 million years ago following a likely rafting event of their own.
Due to this, neither of the Old World "iguanids" are thought to represent basal lineages.
*
Coral snake
Coral snakes are a large group of elapid snakes that can be divided into two distinct groups, the Old World coral snakes and New World coral snakes. There are 16 species of Old World coral snakes, in three genera (''Calliophis'', '' Hemibungarus' ...
s comprise about 16 species in Asia and over 65 species in the Americas. However, none of the American clades are basal, implying that the group's ancestry was in the Old World.
* Extant australidelphian marsupials constitute about 240 species in Australasia and one species (the
monito del monte
The monito del monte or colocolo opossum, ''Dromiciops gliroides'', also called ''chumaihuén'' in Mapudungun, is a diminutive marsupial native only to southwestern South America (Argentina and Chile). It is the only extant species in the ancient ...
) in South America. The fact that the monito del monte occupies a basal position (the most basal species, genus, family and order) in the superorder
Australidelphia
Australidelphia is the superorder that contains roughly three-quarters of all marsupials, including all those native to Australasia and a single species — the monito del monte — from South America. All other American marsupials are members of ...
is an important clue that its origin was in South America. This conclusion is consistent with the fact that the South American order
Didelphimorphia is basal within infraclass
Marsupialia
Marsupials are any members of the mammalian infraclass Marsupialia. All extant marsupials are endemic to Australasia, Wallacea and the Americas. A distinctive characteristic common to most of these species is that the young are carried in a pou ...
; i.e., extant marsupials as a whole also appear to have originated in South America.
* While the bat superfamily
Noctilionoidea
Noctilionoidea is a superfamily of bats containing seven families: Thyropteridae, Furipteridae, Noctilionidae, Mormoopidae, Phyllostomidae, Myzopodidae, and Mystacinidae.
It is one of three superfamilies in the suborder Yangochiroptera, the ...
has over 200 species in the Neotropics,
two in New Zealand, and
two in Madagascar, the basal position of the Malagasy family
suggests, in combination with the fossil record and the next-most-basal placement of the New Zealand family, that the superfamily originated in Africa and then migrated eastward to South America, proliferating there but surviving in the Old World only in
refugia.
* The genus ''
Urocyon
''Urocyon'' (Greek: "tailed dog") is a genus of Canidae which includes the gray fox (''Urocyon cinereoargenteus'') and the island fox (''Urocyon littoralis''). These two fox species are found in the Western Hemisphere. Whole genome sequencing i ...
'' (gray and island foxes) is basal in the
canine subfamily,
suggesting a North American origin of the nearly worldwide group. This is consistent with fossil evidence indicating a North American origin for the
canid family as a whole (the other two canid subfamilies, the extinct
Borophaginae
The extinct Borophaginae form one of three subfamilies found within the canid family. The other two canid subfamilies are the extinct Hesperocyoninae and extant Caninae. Borophaginae, called "bone-crushing dogs", were endemic to North America du ...
and
Hesperocyoninae
The extinct Hesperocyoninae are one of three subfamilies found within the canid family. The other two canid subfamilies are the extinct Borophaginae and extant Caninae.
Taxonomic history
Hesperocyoninae are basal canids that gave rise to the o ...
,
the latter being basal in Canidae, were both
endemic
Endemism is the state of a species being found in a single defined geographic location, such as an island, state, nation, country or other defined zone; organisms that are indigenous to a place are not endemic to it if they are also found elsew ...
to North America).
Notes
References
External links
*
{{Phylogenetics
Phylogenetics