The Questions of Developmental Biology

Such a trait would be selected for in desert environments, where dessication is a constant
risk. As Van Valen reflected in 1976, evolution can be defined as "the control of development by
ecology."
Duplication and divergence
Modularity also allows duplication and divergence. The duplication part of this process
allows the formation of redundant structures, and the divergence part allows these structures to
assume new roles. One of the copies can maintain the original role while the others are free to
mutate and diverge functionally. This can happen at numerous levels. The Hox genes, TGF-β
family genes, MyoD family genes, and globin genes each probably started as a single gene that
duplicated several times. After the duplication, mutations caused the divergences that gave the
members of each family new functions. At the tissue level, one sees duplication and divergence in
the somites that give rise to the cervical, thoracic, and lumbar vertebrae.
Co-option
No one structure is destined for any particular purpose. A pencil can be used for writing,
but it can also be used as a toothpick, a dagger, a hole-puncher, or a drumstick. On the molecular
level, the gene engrailed is used for segmentation in the Drosophila embryo, is used later to
specify its neurons, and is used in the larval stages to provide an anterior-posterior axis to
imaginal discs. Similarly, a protein that functions as an enolase or alcohol dehydrogenase enzyme
in the liver can function as a structural crystallin protein in the lens (Piatigorsky and Wistow
1991). In other words, preexisting units can be co-opted (recruited) for new functions.
Sometimes, whole pathways are co-opted from one system to another. For instance, the pathway
by which Hedgehog protein induces Engrailed protein to pattern and extend the insect wing is
later used within the wing blades to make the eyespots of butterflies and moths. Distal-less,
another protein used to extend the wing imaginal disc, is later used to form the center of such
eyespots (Figure 22.20). Co-option can also be seen on the morphological level. Wings have
evolved three times during vertebrate evolution, and in each case, different forearm structures
were modified for an entirely new function. A structure originally used for walking has been
recruited into a structure suitable for flying.
A famous case of co-option is the use of embryonic jaw parts in the creation of the
mammalian middle ear, as explicated in Chapter 1 (see Figure 1.14; Gould 1990). First, the gill
arches of jawless fishes became the jaws of their descendants; then, millions of years later, the
upper elements of the reptilian jawbone became the malleus and incus (hammer and stirrup)
bones of the mammalian middle ear (Figure 22.21).