Hox genes such as Ubx control where and when structures such as legs form. By tweaking the expression of these genes, the number (or presence) of these structures can be changed from one organism to another. These different organisms can fit into different niches, helping evolution to occur.
Work Step by Step
See. pp. 539-540. Hox genes control developmental programs, and changes in their expression (Fig. 25.25) alter the appearance of the organism in ways that are heritable and which can be acted in by natural selection.