Convergent Evolution: Limited Forms Most Beautiful

Chapter 2. Convergent Animals

2 In all cases of two very distinct species furnished with apparently the same anomalous organ, it should be observed that, although the general appearance and function of the organ may be the same, yet some fundamental difference can generally be detected. I am inclined to believe that in nearly the same way as two men have sometimes independently hit on the very same invention, so natural selection, working for the good of each being and taking advantage of analogous variations, has sometimes modified in very nearly the same manner two parts in two organic beings, which owe but little of their structure in common to inheritance from the same ancestor. —Darwin (1859, 193–194) Locomotion Some of the most spectacular examples of convergent evolution are clearly due to the functional constraints of locomotion. Consider one of the most frequently cited cases of convergent evolution: the astonishing morphological similarity between the extinct Mesozoic marine reptile Ichthyosaurus platyodon and the living marine mammal Phocaena phocaena, the harbor porpoise, or Tursiops truncatus, the bottlenose dolphin. Not only do they look amazingly similar to one another, but they all look amazingly similar to large, fast-swimming fishes like Xiphias gladius, the swordfish, or Carcharodon carcharias, the great white shark. The cartilaginous fishes and the bony fishes both solved the physics of swimming in the dense medium of water back in the Silurian by evolving streamlined, fusiform morphologies (table 2.1). Some 230 million years later, a group of land-dwelling reptiles rediscovered this same morphology in their evolutionary return to the sea. And around 175 million years after that, a group of land-dwelling mammals also rediscovered this same morphology in their own evolutionary return to the sea. Convergent Animals 14 Chapter 2 Table 2.1 Convergent evolution of animal swimming morphologies 1 Convergent structure and function: FUSIFORM BODY (missile-shaped form for drag reduction) Convergent lineages: 1.1 Great white shark (Vertebrata: Chondrichthyes: Elasmobranchii: Lamnidae; Carcharodon carcharias) 1.2 Swordfish (Vertebrata: Osteichthyes: Actinopterygii: Teleostei: Xiphiidae; Xiphias gladius) 1.3 Ichthyosaur (Osteichthyes: Saurcopterygii: Reptiliomorpha: Amniota: Sauropsida: Diapsida: Ichthyosauria: Ichthyosauridae; Icthyosaurus platyodon †Jurassic) 1.4 Harbor porpoise (Amniota: Synapsida: Therapsida: Mammalia: Eutheria: Laurasiatheria: Cetartiodactyla: Cetacea: Odontoceti: Phocaenidae; Phocaena phocaena) 2 Convergent structure and function: EEL-SHAPED BODY (elongated, cylindrical form for drag reduction) Convergent lineages: 2.1 Reed fish (Osteichthyes: Actinopterygii: Cladista: Polypteridae; Erpetoichthyes calabaricus) 2.2 European eel (Actinopterygii: Teleostei: Elopomorpha: Anguillidae; Anguilla anguilla) 2.3 South American lungfish (Osteichthyes: Sarcopterygii: Dipnoi: Lepidosirenidae; Lepidosiren paradoxa) 2.4 Mosasaur (Sarcopterygii: Reptiliomopha: Amniota: Sauropsida: Diapsida: Lepidosauromorpha: Squamata: Scleroglossa: Autarchoglossa: Anguimorpha: Mosasauridae; Platecarpus ictericus †Cretaceous) 2.5 Geosaur (Diapsida: Archosauromorpha: Crurotarsi: Crocodilia: Metriorhynchidae; Geosaurus giganteus †Jurassic) 3 Convergent structure and function: PADDLE-FORM APPENDAGES (walking appendages modified to paddle shapes for paddling/rowing through water) Convergent lineages: 3.1 Sea scorpions (Bilateria: Protostomia: Ecdysozoa: Arthropoda: Cheliceriformes: Merostomata: Eurypterida: Eurypteridae; Eurypterus remipes †Silurian) 3.2 Marbled diving beetle (Arthropoda: Mandibulata: Hexapoda: Dytiscidae; Thermonectes marmoratus) 3.3 Green sea turtle (Bilateria: Deuterostomia: Chordata: Osteichthyes: Sarcopterygii: Reptiliomorpha: Amniota: Sauropsida: Anapsida: Testudines: Chelonioidae; Chelonia mydas) 3.4 Plesiosaur (Sauropsida: Diapsida: Lepidosauromorpha: Sauropterygia: Elasmosauridae; Muraenosaurus leedsii †Jurassic) 3.5 King penguin (Diapsida: Archosauromorpha: Ornithodira: Dinosauria: Saurischia: Theropoda:Maniraptora:Aves:Sphenisciformes:Spheniscidae;Aptenodytes patagonica) 3.6 Africanmanatee(Amniota:Synapsida:Therapsida:Mammalia:Eutheria:Afrotheria: Sirenia: Trichechidae; Trichechus senegalensis) 3.7 California sea lion (Eutheria: Laurasiatheria: Carnivora: Caniformia: Otariidae; Zalophus californianus) Note: The geological age of extinct species is marked with a †. Convergent Animals 15 The evolution of an ichthyosaur or a porpoise morphology is not trivial. It can be correctly described as nothing less than astonishing that a group of land-dwelling tetrapods, complete with four legs and a tail, could devolve their appendages and tails back into fins like those of a fish, while also reevolving a dorsal fin. Highly unlikely, if not impossible? Yet it has happened twice, convergently in the reptiles (sauropsid amniotes ) and the mammals (synapsid amniotes), two groups of animals that are not closely related. We have to go back in time as far as the Carboniferous to find a common ancestor for the mammals and the reptiles; thus, their genetic legacies are very different. Nonetheless, the ichthyosaur and the porpoise both independently reevolved fins. An extremely ancient form of swimming morphology can be seen in some of the first chordate animals to evolve back in the Cambrian, not long after the evolution of animals themselves some 600 million years ago...