Systematics and Evolution 141that does not include all descendants of a common ancestor. To a cladist, the wastebasket
(paraphyletic) family Pongidae is invalid, just as Reptilia without birds is paraphyletic and
invalid. A natural (monophyletic) group would be to place us within the ape family Pon-
gidae or to expand the Hominidae to include most or all of our close ape relatives. In fact,
that is what has happened: most of the great apes are now in the family Hominidae. This
makes a lot of sense logically, but it is tough for many to accept when they were trained to
believe in the Pongidae.
This is the brave new world of cladistic classification. Most classifications of animals and
plants are partly monophyletic (e.g., birds and mammals are natural monophyletic groups)
but mix in a lot of paraphyletic wastebaskets as well (including reptiles and amphibians,
as traditionally defined). Some groups, like the invertebrates, are unnatural by definition,
because invertebrates are defined by the shared primitive lack of a specialized feature, the
FIGURE 5.5. Traditional classifications emphasize the shared primitive similarity of the “great apes” and place
them in their own family Pongidae, while acknowledging the big differences between apes and humans by
placing humans in their own family, Hominidae. However, to a cladist the family Pongidae is paraphyletic
because it does not include all descendants (i.e., humans) of the common ancestor of the human-ape stem. In
this framework, Hominidae must be expanded to include all the great apes (as most anthropologists now agree).
Paraphyletic “PONGIDAE” HOMINIDAEOLD WORLDMONKEYSGIBBONS ORANGUTANGORILLA CHIMPANZEES HUMANSMonophyletic group
(apes + humans)