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 form an outgroup to the rest of the clade.
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Basal group |
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The word "basal" is preferred to the loaded term "primitive", which has false connotations of inferiority or simplicity.
The basal status of a clade is relative to what it is being compared to. In the first example below, humans are the basal group, as they are the first to diverge from the lineages of plant. In the second example, plants are basal to the animals considered.
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Humans |
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Plants |
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The term basal can only be correctly applied to clades, not to traits - although it is frequently misused in the 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: in the example of plants and apes above, it would be likely to break down.
References
Budd, G & Jensen, S (2000). "A critical reappraisal of the fossil record of the bilaterian phyla". Biological reviews 75: 253. doi:.
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