Breaking up a continous phylogeny into discrete taxa is as arbitrary as sawing up a tree. Useful terms are: anagenesis: evolutionary change in an evolving lineage.
cladogenesis: splitting of a lineage into two or more descendent lineages.
grades: taxa based on anagenetic changes that are deemed important enough to seperate descendent from ancestral stacks, e.g. traditional 'reptiles', with birds excluded because of the evolution of many profound changes in structure and physiology associated with powered flight.
monophyletic: a monophyletic taxon includes all descendantsof a single ancestor.
paraphyletic: a paraphyletic taxon includes some but not all of the descendants of a single ancestor.
polyphyletic: a polyphyletic taxon includes descendants of two or more ancestors, both of which have descendants that are not included in the taxon. (Check Figure 3.2, FE p. 48)
Modern classifications derived from cladistic analysis seek to recognise only monophyletic taxa. Paraphyletic taxa, besides being arbitrary because they are based on opinions concerning grade changes, can also be misleading with respect to cladogenetic relationships.