A stem taxon is an extinct species or clade that is related to a group that includes living members. Specifically, based on the provided reference, a stem taxon is defined as an extinct species or clade for which immediate outgroup includes at least one living member.
Understanding Stem Taxa
In the study of evolutionary history and classification (phylogenetics), taxa (groups of organisms) are often divided into stem groups and crown groups. While a crown group includes the most recent common ancestor of all living members of a clade and all of its descendants (living or extinct), a stem taxon represents an extinct lineage that branched off before the origin of the crown group.
Think of it like a tree: the trunk and branches leading up to where the major living branches diverge represent the stem, while the point of divergence of the living groups and everything above that point is the crown.
Key Characteristics from the Definition
Based on the reference provided, the definition highlights crucial aspects of what constitutes a stem taxon:
- Extinct Nature: A defining feature is that a stem taxon is an extinct species or clade. This distinguishes them from crown groups which, by definition, include living members.
- Relationship to Living Outgroups: The definition states that the immediate outgroup includes at least one living member. This means the stem taxon is more closely related to the crown group (which contains living species) than to any other group that includes living species and branched off earlier.
- Node-based Context: The reference also mentions "Node-based definition. statement specifying a clade composed of the most recent common ancestor." While this part might relate more directly to defining the crown group or the node separating stem and crown, it is included in the entry for "Stem taxon," indicating the concept is understood within the framework of phylogenetic tree nodes and common ancestry. A stem group includes all extinct taxa on the lineage between the divergence from the nearest living outgroup and the origin of the crown group.
In essence, stem taxa represent the evolutionary lineages that led towards a modern group but did not survive to the present day. Studying them is vital for understanding the step-by-step acquisition of features that characterize living biodiversity.
