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Developmental modules based on a common set of genetic instructions can evolve separately within a species because genetic switches control how the toolkit is used. These switches include:

Multiple switches control each gene, creating different expression patterns in different locations. In this way, elements of the genetic toolkit can be involved in multiple developmental processes and still allow individual modules to develop and evolve independently.

During evolution, changes in the functions of genetic switches have led to changes in the forms and functions of organisms. To illustrate this, let’s look at the development of wings in Drosophila and other insects. Drosophila species are members of the insect group Diptera, which means “two wings”—that is, they have a single pair of wings, whereas most insects have two pairs of wings (i.e., four wings). The single pair of wings of dipterans develops on the second thoracic segment, where the Hox gene Antennapedia (Antp) is expressed. Antp is also expressed in the third thoracic segment, but in that segment a pair of balancing organs called halteres develops in dipterans. A critical difference between thoracic segments 2 and 3 is that another Hox gene, Ultrabithorax (Ubx), is expressed along with Antp in segment 3 (Figure 19.16). Ubx represses Antp function in dipterans. If Ubx is inactivated by mutation, a second pair of wings forms in thoracic segment 3, as is typical of many other insect groups. Thus some major morphological differences among groups of animals can result from relatively small changes in gene expression.

Question

If Ubx was expressed in all segments, the expression of the wing-forming gene would be inhibited and halteres would form.