Abstract
The concept of common descent, or monophyly, is held by modern biologists as a "near ideological commitment" (p. 332), says Malcolm Gordon, professor of biology at UCLA. In this remarkable essay, Gordon looks at two episodes in the history of life-the origin of life itself and the origin of tetrapods-and argues that in both cases, the evidence favors multiple, or polyphyletic, origins. The standard concept of common ancestry is well supported only at the smaller scales of taxonomic differentiation, Gordon asserts, namely, "from populations to the levels of genera and families" (p. 333), although he adds that "there are no single, universal scenarios, even at these levels."