scientist with beakers
In biology and biological taxonomy, a clade is a group consisting of a single common ancestor, all descendants of that ancestor, and nothing else. Over centuries of work, biological taxonomy has striven to divide groups into clades, rejecting classifications that are not clades, which are called “paraphyletic”. The true clades are “monophyletic”.
An example of a true clade would be birds. All birds are believed to be descended from a common ancestor that lived about 150 million years ago. However, reptiles and monkeys are not clades. Reptiles are not a clade because birds are descended from dinosaurs, considered reptiles, and birds are not considered reptiles. A group that excludes descendants of a common ancestor is not a clade. Apes are not a clade because humans are descended from apes and humans are not generally considered apes. If you include humans and extinct relatives of humans, such as Neanderthals, such as apes, then apes are a clade, but this is generally not done.
Simpler organisms like arthropods (crustaceans, insects, millipedes, etc.) are more difficult to organize into clades because there are fewer genetic and morphological traits that can be used to determine common ancestry and ancestral lineages. For example, for decades in the late 20th century, scientists thought that arthropods (animals with external exoskeletons and jointed appendages) evolved at several different times from soft-bodied ancestors such as annelid worms. Later morphological and genetic analyzes found this to be false: the arthropods are, in fact, a clade, descended from a common ancestor that diverged from the soft-bodied ancestors only once.
Determining clades at levels more specific than phyla can be challenging, especially for relatively simple animals. After decades of study, we still don’t know how different groups of arthropods are related to each other. Did terrestrial arthropods evolve from magic shrimp or some other group? We’re not sure, and scientists are busy publishing papers and running analyzes to find out.
Determining clades is difficult in part because much of the morphological and genetic data is ambiguous. Sometimes a certain morphological feature, such as spines, evolves through parallel evolution rather than manifesting in a single species and most (or all) of its descendants. Genetic data can be ambiguous because evolution occurs in different species at different rates, ruling out calculations that attempt to date the timing of divergence between species by comparing genetic similarity. To make matters worse, morphologists and geneticists tend to argue about the relative importance of their respective approaches. Correct determinations about animal clades only emerge after years or decades of in-depth research representing hundreds or thousands of articles and studies.