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. 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]

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

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:

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

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]

' Basal angiosperms '

Great apes

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Within the

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

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.

• Spiders of the genus Amaurobioides are present in South Africa, Australia, New Zealand and Chile.^[22][23] 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.^[23]
• rafting event.^[26][27] However, a 2022 study found oplurids to be closely allied with the American iguanians Leiosauridae, having only diverged 60 million years ago following a likely rafting event of their own.^[28]
  Due to this, neither of the Old World "iguanids" are thought to represent basal lineages.
• Coral snakes 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.^[29]
• Extant australidelphian marsupials constitute about 240 species in Australasia and one species (the
  Marsupialia; i.e., extant marsupials as a whole also appear to have originated in South America.^{[18][n 10][n 11][n 12]}
• While the bat superfamily
  two in Madagascar, the basal position of the Malagasy family^[36] 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.^[37]
• The genus
  endemic
  to North America).