Biological taxonomy aims to divide groups into clades, which consist of a common ancestor and all its descendants. Non-cladic classifications are rejected. Arthropods are difficult to organize into clades, but they are a true clade descended from a common ancestor. Determining clades is challenging due to ambiguous morphological and genetic data.
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 tried to divide groups into clades, rejecting non-cladic classifications, which are defined as “paraphyletic”. True clades are “monophyletic”.
An example of a true clade would be birds. It is believed that birds all descended from a common ancestor who lived about 150 million years ago. However, reptiles and monkeys are not clades. Reptiles are not a clade because birds are descended from dinosaurs, which are 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 generally not considered apes. If you include humans and extinct relatives of humans, such as Neanderthals, as apes, then apes are a clade, but it’s generally not done.
Simpler organisms, such as arthropods (crustaceans, insects, millipedes, etc.) are more difficult to organize into clades, because there are fewer genetic and morphological features that can be used to determine common ancestors and ancestral lineages. For example, for decades in the late 20th century, scientists thought that arthropods (animals with external exoskeletons and jointed appendages) evolved on separate occasions from soft-bodied ancestors such as annelid worms. Subsequent morphological and genetic analyzes found this to be false: Arthropods are really a clade, descended from a common ancestor that split from soft-bodied ancestors only once.
Determining clades at more specific levels of phyla can be challenging, especially for relatively simple animals. After decades of study, we still don’t know how different arthropod groups are related to each other. Did terrestrial arthropods evolve from fairy shrimp or some other group? We don’t know for sure, and scientists are busy publishing papers and conducting analyzes to find out.
Clade determination is difficult in part because much of the morphological and genetic data is ambiguous. Sometimes, a certain morphological feature – such as spines – evolve through parallel evolution rather than manifesting itself 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, nullifying calculations that attempt to date the time of divergence between species by comparing genetic commonality. To make matters worse, morphological specialists and geneticists tend to argue over the relative meaning of their respective approaches. Correct animal clade determinations emerge only after years or decades of extensive research representing hundreds or thousands of articles and studies.
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