All insect species on Earth have exactly six legs—a pair on each of three thoracic (middle) body segments. Look at the larger group of arthropods, however, and you will see a striking variation in leg number, including finding legs on abdominal segments. These dramatic differences in morphology represent changes in self-contained body units (modules). The modular changes can arise through relatively small changes in key regulatory genes. In this tutorial we will examine an example of such modular changes in insects. We will also see how relatively small changes in the timing or place of expression of key regulatory genes can affect the morphology of different species—in this case comparing the hindlimbs of ducks (webbing) to those of chickens (no webbing).
Modular changes in organisms have the potential to create dramatic changes in a species. Because modules are self-contained units, changing a module often has no deleterious affect on the functioning of the body as a whole. If a module can change without significant harm to the organism, over the course of evolution species can change the identity of individual modules, resulting in better (or worse) adaptation of the species to its environment. Through understanding the roles of key developmental regulatory genes, scientists are learning that many of these dramatic changes in morphology can arise through relatively small changes in genes.
Textbook Reference: Concept 14.4 Changes in Gene Expression Pathways Underlie the Evolution of Development