User:Manudouz/sandbox/Basal

In phylogenetics, basal refers to the position of a node or a branch with respect to the base (or root) of a rooted phylogenetic tree or cladogram.
 * A basal node is directly adjacent (in a strict sense) or close (in a more loosely applied sense) to the root.
 * A basal branch connects an internal node with the root, as opposed to a terminal branch which connects an internal node with a tip.

The term can be extended to clades because each internal node in a tree corresponds to the last common ancestor (LCA) of a clade. For example, clade C may be described as basal within a larger clade D if its LCA is directly linked to the LCA of D. The terms deep-branching or early-branching are similar in meaning.

'Basal' in paleontology: "Describe relationships on the basis of sister-group relationships and/or following the hierarchical pattern of cladograms: ‘the new mammal presented here is the sister taxon of all other therians’ (rather than: the ‘most basal’ Theria); ‘our analysis indicates that the new taxon belongs to Dinosauriformes but outside Dinosauria’ (rather than: a new basal dinosauriform)."

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. 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.