Clades

Clades are fundamental units in cladistics. A clade is a group that includes an ancestral species and all its descendants, forming a monophyletic lineage. In a phylogenetic tree, clades correspond to branches that can be traced to a single common ancestor. Clades are defined by shared derived characters, or synapomorphies, that unite all members and exclude any organisms not descended from that ancestor.

Determining clades relies on reconstructing evolutionary relationships using data such as DNA sequences and morphology. Outgroups are used to polarize traits and root trees, enabling the identification of the most recent common ancestor of a group. The result is a hierarchical map of relationships in which clades nest within larger clades.

Within classifications, clades are preferred as units because they reflect evolutionary history. A monophyletic group contains the ancestor and all descendants. A paraphyletic group includes the ancestor and only some of its descendants (for example, the traditional grouping Reptilia excluding birds). A polyphyletic group comprises taxa whose last common ancestor is not included within the group, often assembled on the basis of superficial similarity rather than shared ancestry.

Cladistics emphasizes monophyly as a criterion; groups that do not meet this are often revised. Molecular data have strengthened clade definitions and resolved deep relationships, though rapid radiations and horizontal gene transfer can complicate inference. The concept of clades underpins the tree of life and informs taxonomy, comparative biology, ecology, and evolutionary studies by organizing biodiversity according to evolutionary history rather than superficial similarity.