Basal (phylogenetics)
In phylogenetics, basal is the direction of the base 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, species diversity, 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 of that rank within D. The concept of a 'key innovation' implies some degree of correlation between evolutionary innovation and diversification. 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 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 of lowest rank within a larger clade; e.g., core eudicots.
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 assumed in this example that the terminal branches of the cladogram depict all the extant taxa of a given rank within the clade; otherwise, the diagram could be highly deceptive. Additionally, this qualification does not ensure that the diversity of extinct taxa is represented.
In phylogenetics, the term basal can 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" are preferred to "basal" or "primitive", the latter of which may carry false connotations of inferiority or a lack of complexity.
Despite the ubiquity of the usage of basal, some systematists believe its application to extant groups is unnecessary and misleading. The term is more often applied when one branch is less diverse than another branch. 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 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. Other famous examples of this phenomenon are the oviparous reproduction and nipple-less lactation of monotremes, a basal clade of mammals with just five species, and the archaic anatomy of the tuatara, a basal clade of lepidosaurian with a single species.
Examples
Flowering plants
The flowering plant family Amborellaceae, restricted to New Caledonia in the southwestern Pacific, is a basal clade of extant angiosperms, consisting of the most basal species, genus, family and order within the group. 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, 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 features that have only been sorted out via comparison with other angiosperms and their positions within the phylogenetic tree. The cladogram below is based on Ramírez-Barahona et al., with species counts taken from the source indicated.Great apes
Within the great apes, gorillas are a sister group to common chimpanzees, bonobos and humans. These five species form a clade, the subfamily Homininae, of which Gorilla is the basal genus. However, if the analysis is not restricted to genera, the Homo plus Pan clade is also basal.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.
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.
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 is basal, or branches off first, within another group may not make sense unless the appropriate taxonomic level is specified. If that level cannot be specified a more detailed description of the relevant sister groups may be needed.
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, a suggestion that is consistent with other evidence. Orangutans also differ from African apes in their more highly arboreal 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 in a widely dispersed taxon or clade can provide valuable insight into its region of origin. In some situations where it might not otherwise be obvious, the direction of migration away from the area of origin can also be inferred. Examples include:- Spiders of the genus Amaurobioides 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 lizards are distributed throughout the Americas, on Madagascar, and on Fiji and Tonga in the western South Pacific. The Malagasy forms are 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 suggests that iguanids once had a widespread Gondwanan distribution; after the Malagasy and New World representatives were separated by vicariance, less isolated Old World iguanids became extinct through competition with other lizard groups. 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.
- 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.
- Extant australidelphian marsupials constitute about 240 species in Australasia and one species in South America. The fact that the monito del monte occupies a basal position in the superorder Australidelphia 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; i.e., extant marsupials as a whole also appear to have originated in South America.
- While the bat superfamily Noctilionoidea 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 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.