Figure 14.21: Parallel Phenotypic Evolution in Sticklebacks A developmental gene, Pitx1, encodes a transcription factor that stimulates the production of plates and spines. This gene is active in marine sticklebacks (indicated by arrowheads in inset at left) but is mutated and inactive in various freshwater populations of the fish. The fact that this mutation is found in geographically distant and isolated freshwater populations is evidence for parallel evolution.