How do scientists use clades to classify chordates and vertebrates?

Scientists classify chordates and vertebrates using phylogenetic taxonomy, also known as cladism, which seeks to organize organisms into clades. A clade is a monophyletic group consisting of a common ancestor and all of its descendants.1

In this system, classification is based on synapomorphies—uniquely shared derived characteristics that indicate a common evolutionary history. Scientists identify these traits by comparing a group of organisms against an "outgroup," which is a related species that branched off earlier in the evolutionary tree. If a character is present in the study group but absent in the outgroup, it is considered a derived trait that defines the clade.2

This approach has significantly changed how chordates are categorized:
* Monophyletic Groups: Modern systematics prioritizes clades over "grades" (informal groups based on physical similarity). For instance, while Vertebrata is a valid clade, the term "invertebrate" is merely a grade and not a formal taxonomic group.1
* Revising Traditional Groups: Cladism has led to the abandonment of paraphyletic groups. For example, because birds are modified dinosaurs, Aves is a clade, whereas the traditional class Reptilia is often considered a grade because it excludes birds. Similarly, the old class of fishes (Pisces) is rarely used because birds, mammals, and amphibians are all modified descendants of fish lineages.1