Why are invertebrates not considered a single natural group?

In zoology, invertebrates are not considered a single natural group because they do not form a monophyletic clade—a group that includes a common ancestor and all of its descendants. Instead, the term "invertebrate" is defined solely by the absence of a vertebral column, a negative characteristic that encompasses an immense diversity of life.12

Because this classification is based on what these animals lack rather than shared evolutionary history, invertebrates are distributed across more than 30 distinct phyla. In contrast, all vertebrates belong to a single phylum, Chordata, which also includes some invertebrate groups.2

From the perspective of modern phylogenetic taxonomy, which seeks to classify organisms based on shared evolutionary history and synapomorphies, the category of "invertebrate" is paraphyletic. It artificially groups together unrelated lineages that happen to lack a backbone, failing to reflect the complex, branching evolutionary relationships that define the tree of life.132