The Questions of Developmental Biology

Scientists are looking at co-option in the formation of novel evolutionary structures.
For instance, the carapace (dorsal shell) of the turtle is an evolutionary novelty that appears to
form in a manner reminiscent of limbs. There is even a carapacial ridge that organizes the
mesenchyme much like the apical ectodermal ridge of the limb bud (Figure 22.22; Burke 1989a).
The bones themselves appear to form in the manner of skull bones. It is possible that certain
developmental pathways (those used to form the limbs and those used to form the skull bones)
have been recruited to form this new structure. The existence of discrete developmental modules
allows the principles of dissociation, duplication and divergence, and co-option to form new types
of organisms.