Threespine stickleback fish have independently lost their pelvic girdle across several freshwater lakes. While it was long thought that selection favored the loss of this trait, it was recently shown that mutation also plays a surprising role. The regulatory region of the Pitx1 gene has a high probability of breaking due to torsion during DNA replication, thus it is called a “fragile site”. This breakage drives elevated rate of mutation at the locus, with many mutant alleles of Pitx1 having low (or no) gene expression only around the pelvic region. We’ll call this no-pelvis mutant allele “-”, and the wildtype allele “+”. Assume the following: