Clade

A clade is a group consisting of a common ancestor and all its descendants. Its members are linked by shared evolutionary ancestry.

Phylogeny: The pattern of evolutionary relationships among organisms or other biological entities. A clade is a branch within a phylogeny.
Synapomorphy: A shared, derived character inherited from the most recent common ancestor of a group. Synapomorphies help identify and diagnose clades.
Paraphyly: The condition of a group containing a common ancestor but excluding one or more descendants. A paraphyletic group fails the all-descendants rule that defines a clade.
Phylogenetic nomenclature: A system of naming taxa by reference to their placement on a phylogenetic tree. It defines many named groups directly as clades.
Incomplete lineage sorting: The persistence of ancestral genetic variation across successive speciation events, producing gene trees that differ from species trees. It can make the ancestry of closely spaced branches difficult to resolve.
Common descent: The principle that organisms descend from shared ancestors over generations. Shared descent is the ancestry that unites every member of a clade.
Cladistics: A method of classifying organisms by branching evolutionary relationships and shared derived characters. Cladistics seeks groups that correspond to clades.
Polyphyly: The condition of a group assembled from lineages whose most recent common ancestor is excluded. A polyphyletic grouping does not represent a single complete ancestral branch.
Phylogenetic comparative methods: Statistical methods that account for shared ancestry when comparing species. Clade relationships help prevent ancestry from being mistaken for independent evidence.
Horizontal gene transfer: The movement of genetic material between organisms outside parent-to-offspring inheritance. It can produce reticulate histories that a simple branching clade model cannot capture.

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