Biological taxonomy categorizes species into monophyletic and polyphyletic groups. Genetic methods have made it easier to distinguish between superficially similar groups, such as legless lizards and snakes. Warm-blooded animals are a polyphyletic group, including birds and mammals. Taxonomists aim for specific classification, unlike casual observations that may group unrelated species together.
In biological taxonomy – also called scientific classification or biological classification – monophyly means that a group exclusively includes one species and all of its ancestors, while polyphily means that a group may contain a “baggage” of several families. These are called monophyletic and polyphyletic groups, respectively. An example of a polyphyletic group would be “worms” or “warm-blooded animals”, while a monophyletic group would be “mammals” or “crustaceans”.
The history of biological taxonomy has been to try to eliminate polyphyletic groups in favor of monophyletic ones. Since the 1970s, this has been made much easier by genetic methods – also called phylogenetic analysis or “molecular studies” – which study similar lengths of DNA to find out how animals are related to each other. Many groups that appear superficially similar can turn out to be completely unrelated in practice. For example, Pygopodidae, a family of legless lizards, appear similar to snakes but are distinguished from them by flashing eyelids (which snakes lack), external earholes, flat, unforked tongues, and limbs vestigial. For an amateur, telling the difference can be a little difficult, but for a professional biologist, it can be clear.
One of the most standard examples of a polyphyletic group are warm-blooded animals, which include both birds and mammals. Both species have a common ancestor who lived during the Paleozoic era, long ago. Birds, though warm-blooded, evolved from cold-blooded ancestors, dinosaurs, which are hardly monophyletic with mammals. Thus birds and mammals belong to completely different groups, but both fall into the general category of warm-blooded animals.
Specific biological taxonomy is the goal of taxonomists, but it often runs counter to common wisdom or lack of rigor. For example, when asked “Are there marine insects?” some might answer, “Well, sure, lobsters are a bit like bugs.” While to the casual observer this answer might seem like enough, it’s enough to make a career taxonomist practically spit the coffee.
Insects are members of the class Insecta, while lobsters are members of the subphylum Crustacea, an entirely different group. While both are arthropods and likely related, the groups are quite different, one primarily terrestrial and the other aquatic. Casually referring to them as a monophyletic group is the kind of habit taxonomists are fighting against by being more specific about the relationships between animals, which helps us understand them better.
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