Basal (phylogenetics)

In phylogenetics, basal is the direction of the base (or root) of a rooted phylogenetic tree or cladogram. Clade C may be described as basal within a larger clade D if its root is directly linked (adjacent) to the root of D. If C is a basal clade within D that has the lowest taxonomic rank of all basal clades within D, C may be described as the basal taxon of that rank within D. While there must always be two or more equally basal clades sprouting from the root of every cladogram, those clades may differ widely in rank or species diversity. 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 more 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.

A basal group forms a sister group to the rest of the larger clade, such as in the following example:

 Basal group

 Non-basal group #1

 Non-basal group #2

 Non-basal group #3

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.

The term basal can be correctly applied to clades of organisms, but not to lineages or to individual traits possessed by the organisms—although it may be misused in these ways in technical literature. While "basal" applies to clades, characters or traits are usually considered derived if they are absent in a basal group, but present in other groups. This assumption only holds true if the basal group is a good analogy for the last common ancestor of the group. "Ancestral" or "plesiomorphic" (but not "basal", see Criticism) are preferred to "primitive", which may carry false connotations of inferiority or a lack of complexity.

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