The knifefish, rather than having several fins like a trout, has one long “ribbon fin” that undulates along the length of its body. Studies of its motion reveal that it uses the optimal wavelength to get the most forward thrust, stability, and maneuverability out of its investment of energy. But the knifefish is not alone: the same optimal design can be found in cuttlefish (cephalopods), rays (cartilaginous fish), certain flatworms, and other bony fish that are evolutionarily unrelated. In fact — if evolution by natural selection is assumed to be the cause — this design emerged independently at least eight times. To the Darwinian, it’s another remarkable case of “convergent evolution.” … Casey Luskin has argued that Darwinians appeal to convergence in order to have it both ways: basically, “biological similarity implies common ancestry, except when it doesn’t.” Read the rest on Evolution News & Views.