The three-spined sticklebаck (Gаsterоsteus аculeatus) is a small fish fоund in bоth marine and freshwater environments. Marine stickleback populations consist mostly of individuals with pronounced pelvic spines, as shown in Figure 1. Individuals in freshwater stickleback populations, on the other hand, typically have reduced pelvic spines, as shown in Figure 2. Each figure shows an image of a stickleback fish with a genetic structure below it. The left figure is labeled Figure 1. Marine stickleback. A long Pelvic Spine on the fish is labeled. The genetic structure below the fish contains three enhancers, a promoter, and a gene. From left to right, the Enhancer Sequences are labeled Hindlimb, Pituitary, and Jaw. To the right of the Enhancer Sequences is a Promoter with an arrow moving up and to the right, over the top of the Pitx1 gene. The right figure is labeled Figure 2. Freshwater stickleback. A short Pelvic Spine on the fish is labeled. The genetic structure below the fish contains three enhancers, a promoter, and a gene. From left to right, the Enhancer Sequences are labeled Hindlimb, Pituitary, and Jaw. The Hindlimb enhancer is crossed out with an X, and it is labeled Disabled Due to Mutation. To the right of the Enhancer Sequences is a Promoter with an arrow moving up and to the right, over the top of the Pitx1 gene. As represented in Figure 1 and Figure 2, the phenotypic difference between marine and freshwater sticklebacks involves Pitx1, a gene that influences the formation of the jaw, pituitary gland, and pelvic spine. Enhancer sequences upstream of the Pitx1 genetic locus regulate expression of the Pitx1 gene at the appropriate times and in the appropriate tissues during development. Previous studies have found that a mutation in the hindlimb enhancer interferes with the formation of a pronounced pelvic spine. In sticklebacks, which of the following is most likely to occur if the jaw enhancer is disabled instead of the hindlimb enhancer?