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 the only clade of a given
taxonomic rank that is basal 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 and/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.