In phylogenetics, a basal clade is the earliest clade to branch in a larger clade; it appears at the base of a cladogram.
A basal group forms an outgroup to the rest of the clade, such as in the following example:
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The word "basal" is preferred to the term "primitive", which may carry false connotations of inferiority or a lack of complexity.
The term basal can only be correctly applied to clades of organisms, not to individual traits possessed by the organisms—although it can be misused in this manner in technical literature. While the term "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.
As an example, the flowering plant family Amborellaceae is considered the most basal lineage of extant angiosperms.
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In animal family Hominidae, the gorillas are an outgroup to chimpanzees, bonobos and humans. These four species form a clade, the subfamily Homininae, of which gorillas are the basal member.
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However, in the family Hominidae, the orangutans form an outgroup to the subfamily Homininae, the clade to which gorillas, chimpanzees, bonobos and humans all belong.
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