Paedomorphosis, the evolutionary process in which juvenile stages in the ontogeny of ancestors become the adult stages

© 1996 Journals of Reproduction and Fertility 1359-6004/96 $8.50 Paedomorphosis, the evolutionary process in which juvenile stages in the ontogeny of ancestors become the adult stages of descendants, has occurred frequently in all animal phyla. It can happen as the result of two processes: neoteny, in which the ontogeny of somatic features is retarded so that sexual maturity occurs while juvenile features are retained; and progenesis, in which sexual maturation is accelerated with respect to somatic development. The difference between these two processes, and the relative importance of each, has frequently fuelled debate among evolutionary biologists (for example, de Beer, 1958; Gould, 1977). Neoteny is not uncommon in vertebrate phylogeny. All vertebrates may be neotenous descendants of a primitive chordate in which the sessile adult stage, which still exists in tunicates, is lost (Berrill, 1955), and it has been argued that humans are neotenous descendants of apes (Bolk, 1926). One group of birds is also thought to have passed through a neotenous stage in their evolution the ratites. There are living ratites in all of the southern hemisphere continents: the ostrich (Struthio camelus) from Africa, two species of rhea (Rhea) from South America, the emu (Dromaius novaehollandiae) from Australia, three species of cassowary (Casaurius) from New Guinea and northeastern Australia and three species of kiwi (Apteryx) from New Zealand. Many extinct species have been recorded including moas and elephant birds. Contrary to earlier beliefs (for example, Young, 1962), DNA–DNA hybridization studies suggest that the ratites comprise a monophyletic taxon (Sibley and Ahlquist, 1981). A divergence occurred among them about 80 million years ago, when the southern continents drifted apart, leading to the ostrich and rheas, which themselves soon diverged, and to the Australasian species. A surprising conclusion from this is that the kiwis, which are small, nocturnal and insectivorous, with a long bill and small eyes, diverged more recently from the emu and cassowaries than did the ostrich and rheas, despite the huge size and other similarities between these other species. The kiwis may have undergone a secondary reduction in size, and Gould (1991) has suggested that this may account for the huge size, in relation to body size, of kiwi eggs. All ratites are flightless. Ostriches and rheas have short wings which are used in display. Cassowaries and emus have only vestiges of wings, and in kiwis the remnants of wings are barely detectable. The ratites have no keel to the sternum (hence ratite from the Latin ratis for raft) to which the flight muscles of flying birds are anchored. Their flightless condition had led them to be regarded as primitive birds. However, de Beer (1956) argued that many of their apparently primitive characters were in fact juvenile rather than primitive, and that, in fact, ratites were neotenous descendants of flying birds. Of particular importance was the arrangement of the palate, which is palaeognathous. Most birds are neognathous, but pass through a palaeognathous stage in their development. Other juvenile characters are the structure of the head with very large eyes (except in kiwis), comparatively long legs (O’Connor, 1984) and downy feathers. In short, adult ostriches, for example, are simply overgrown chicks; although they grow to an enormous size, their morphology remains surprisingly unchanged. True adult avian features never develop. In de Beer’s words, they are the Peter Pans of the avian world.