by Greg Mayer

There’s been some interesting discussion in the comments on the post on Change we can believe in concerning gradual and punctuational evolution, including the question of whether Stephen Jay Gould ever advocated macromutation. (Among his many accomplishments, Gould joined Niles Eldredge in the explication and elaboration of Eldredge’s initial suggestion of the idea of punctuated equilibria.) Over a very productive 30+ year career, Gould’s views were of course not static, but in a 1980 paper (Is a new and general theory of evolution emerging? Paleobiology 6:119-130) he does advocate macromutation:

Instead, I envisage a potential saltational origin for the essential features of key adaptations. Why may we not imagine that gill arch bones of an ancestral agnathan moved forward in one step to surround the mouth and form proto-jaws?

Gould refers here to the serial homology of vertebrate jaws to the gill skeleton, the discovery of which is one of the triumphs of classic comparative anatomy. He proposes jaws to have arisen from the gill skeleton in a single mutation (i.e. a macromutation). There is always the problem that what’s a big mutation to one person is not big to another, but I think most or all vertebrate morphologists would consider the conversion of a gill arch into a jaw in one step a macromutation. In context, Gould is not arguing that all the features of jawed vertebrates would have arisen at once, but that a very major feature would have. Two arches are involved in the jaws: the mandibular arch, forming the jaws themselves, and the hyoid arch, which supports and suspends the mandibular.  Since there are extinct fishes (the acanthodians) whose hyoid arch is little modified from a gill arch, both of the arches involved in jaws did not change in one step. The exact way in which jaws arose is not known, and is the subject of continuing  anatomical, developmental, molecular genetic, and paleontological research.  New fossils from the Chengjiang Lagerstatte in China are beginning to throw more light on early vertebrate evolution, but are a bit early for the origin of jaws; we may hope, and predict, that further discoveries will shed more light on the origin of jaws.