818 THE STRUCTURE OF EVOLUTIONARY THEORY
9 - 12A. The famous figure from Westoll (1949) showing rapid morphological change early in the
history of lungfishes, followed by prolonged stagnation thereafter.branching off of a descendant, leaving a persisting and unaltered ancestor. They
contrast this mode with bifurcation—the style of speciation predicted by
gradualism: splitting of an ancestral population into two descendant species, both
diverging steadily from the ancestor (which becomes extinct). I follow Wagner and
Erwin's restricted use of "cladogenesis" only in discussing their work, and use the
broader definition throughout the rest of this book.
The last two modes of bifurcation and cladogenesis both depict branching
speciation in the definitional sense that two species emerge, where only one existed
before. But note the crucial difference: bifurcation represents the operation of
speciation in a gradualistic world, where an event of branching may be considered
equivalent to two cases of gradualism following a separation of populations, and
where the separation itself need not correlate with any acceleration in rate of
evolutionary transformation. Cladogenesis, on the other hand, represents the
predictions and expectations of punctuated equilibrium. Therefore, if we can model
the differences between bifurcation and cladogenesis, and test these distinctive
expectations against real patterns in nature, we may achieve our best and fairest
potential evaluation for the relative frequency of punctuated equilibrium—for
punctuated equilibrium can-