Read more: 鈥�Rewriting the textbooks: When science gets it wrong鈥�
Vertebrates used to be so simple. They came in five common-sense categories: amphibians, birds, fish, mammals and reptiles. Birds were the winged and feathered ones, reptiles the scaly, cold-blooded ones. And so on. A place for everything and everything in its place.
That was before , a more rational system of taxonomy initiated by the German entomologist Willi Hennig in the 1960s. It analyses shared characteristics and genetic relationships to group species according to their evolutionary ancestry. That sounds fair enough 鈥� but strict rationality, it turns out, plays havoc with those familiar groupings.
Advertisement
A bright spot is that mammals come through fine: a single ancestral species gave rise to all living and extinct mammals and nothing else. That makes them a logical 鈥渃lade鈥� that branches away from the rest of the evolutionary tree at a clearly defined point. Birds survive too.
But pity the poor reptiles. The traditional group Reptilia 鈥� things like lizards, crocodiles, snakes, tortoises plus many extinct groups 鈥� is not a true clade, because the common ancestor of all those animals also gave rise, at different points, to mammals and birds. According to cladistics, you can clump these three groups together in a mega-grouping, known as the amniotes, but you cannot hack off a single, consistent reptile branch (see diagram).
Amphibians fare a little better, but only the living ones: frogs, toads, newts, salamanders and worm-like caecilians. Include the extinct ones, and you encounter the reptile problem on bigger scale: the relevant clade includes all tetrapods, the four-limbed vertebrates. And don鈥檛 even start on fish.
If you think this is cladistic correctness gone mad, you have a point. For everyday purposes, most biologists are happy to use traditional, common-sense classifications based on obvious characteristics. You won鈥檛 ever hear them refer to 鈥渘on-avian, non-mammalian amniotes鈥� when 鈥渞eptiles鈥� would do. But the term is really a hangover from a less well-informed age.
Read previous article: 鈥�Rewriting the textbooks: The periodic turntable鈥�
Read next article: 鈥�Rewriting the textbooks: Confusion over nuclear fission鈥�
