Irreversibility: A Comment on Macbeth's Interpretations

Abstract

Macbeth (1980), writing about irreversibility, found a conflict between Lande (1978) and Wright's (1968) views and those of Simpson (1953) and Laurent (1964), although he stated from the beginning that the authors concern themselves with two different phenomena. The first is that evolution never goes into reverse in a big way; the second that evolution frequently goes into reverse in a small way. I think the purported conflict does not really exist. Lande's model actually belongs to the second category of simple reversibilities. Indeed, complex organs may well reappear sometimes, if their former disappearance resulted from the action of a suppressor regulatory gene(s), that in turn disappears. The catch is that the structure had not really disappeared; it was still there, genetically. Two herpetological examples can be mentioned in this respect. Frogs and toads have no teeth on the lower jaw. However, some have odontoids, which is in line with Dollo's law. More unexpected is the case of Amphignathodon, a genus of marsupial frogs, which has real teeth, in spite of its close relationships with some species of Gastrotheca like G. cornuta. The most likely explanation is the lifting of a very old inhibition. According to Walls' (1942) theory of the origin of snakes, the eyes have been atrophied in their fossorial ancestors, only to reappear, although somewhat different in the flourishing descendants. Again, the suppression of a suppressor is a likely explanation. When it comes to the really big irreversibility-the unreeling of a movie filmwhich is a far cry from the reappearance of a lost organ (however complex), I can only reassert its high degree of improbability. An original limb structure can reappear without posing an intractable problem, but a return from Equus to Hyracotherium through Pliohippus, Merychippus, Miohippus, Mesohippus, Epihippus and Orohippus is a practical impossibility, whether the mechanisms are neatly genetic or hazily epigenetic.