Body Plans and Taxonomic Levels

Body plans are easy to exemplify but difficult to define. It is generally accepted that higher taxa such as phyla and classes are characterized by unique body plans, while lower ones, such as genera and species, are not. Frequent references are made to, for example, the vertebrate body plan, the insect body plan, the molluscan body plan, and so on. But few biologists would consider a pair or group of congeneric species as differing in body plans – rather such species typically share their body plan with each other and with numerous other close relatives.

Let us consider one example of closely-related species in each of the three above-mentioned higher taxa. The genus Geospiza (‘Darwin's finches’) contains several species and has been the subject of considerable study over many years (see Lack (1947) and Grant (1986) for reviews). These, like other birds, clearly exhibit both vertebrate and avian body plans; we do not generally think of a geospizid ‘plan’. The flour beetles Tribolium confusum and T. castaneum – much studied from an ecological perspective (Park 1948,1954, 1962) – are characterized by insect and coleopteran body plans rather than a specifically Tribolium one. The land snails Helix aspersa and H. pomatia are typical gastropod molluscs in their body plan (see Kerney and Cameron 1979 for descriptions), and while they belong to the family Helicidae, we would not so readily categorize their body plan as being a helicid one.