The term clade means "branch". A clade is a phylogenetic group or lineage of organisms, defined as any species and all of it's descendents. (If the species did not have any descendents then it is the sole member of the clade). The clade is the basic and sole unit of the Cladistic system (phylogenetic systematics), in the same way that the taxonomic heirarchy of kindom, phylim, class, etc is for the Linnean system.
Every clade is, by definition, monophyletic. All it's members share at least one (if not more) common inherited (or "derived") characteristics. These characteristics are called "synapomorphies". A clade cannot give rise to descendents that do not belong to that clade. So vertebrates represent a clade, but reptiles do not (because some early reptiles evolved into mammals and birds).
All the members of a clade can be traced back to a hypothetical Most Recent Common Ancestor (sometimes abbreviated to MRCA). This strangely is never actually described. It is simply the hypothetical earliest organism that shared the characterics that define that particular clade.
A broader, non cladistic, definition of a "clade" is any branch on the evolutionary tree, any unique new evolving type which eventually becomes a whole group (such a clade does not have to be monophyletic. In his novel Schizmatrix, the science fiction writer Bruce Sterling (one of the founders of the Cyberpunk genre) writes about a future in which humankind has split into two distinct branches, the Shapers and the Mechanists (according to whether they use biological or mechanical technology). Of course H. G. Wells in his seminal work The Time Machine goes further and imagines a distant future where the human race as having evolved into two distinct species - the gentle surface dwelling Eloi, the descendents of the effete upper classes, and the sinister Morlocks (descendents of the lower classes), who live in tunnels under the ground and emnerge at night to feed on the Eloi. Wells actually developed this story from his early childhood experiences where he lied with his mother who was a servent in a large mansion; the servants lived and worked in tunnels under the house and would go up to the surface to wait on the owners and their guests.
page historypage uploaded 27 December 1999