1.5 Clades and mammals
Are the mammals a clade?
Yes, despite Simpson's earlier reservations about their possible polyphyletic origins, morphological and molecular data now strongly suggest that they are all indeed derived from a single ancestral mammalian species. So the mammals are both a clade and a grade grouping (as are also the birds).
If the taxonomic hierarchy is to give an unambiguous reflection of phylogenetic relationships, then the recognition of clades is the most desirable objective of systematics: members of any taxon, so recognised, will be more closely related to each other than to any member of any other taxon, by definition. A classification based purely on a hierarchy of clades is the objective of cladistics.
What aspect of evolutionary pattern is missing from such a scheme of classification?
Because cladistic hierarchies reflect only increasing levels of inclusiveness of the branchings in a phylogeny, they cannot reflect the different amounts of evolutionary change between ancestral and descendent organisms. In other words, they ignore the anagenetic component of pattern, upon which grades are based.