Homeobox genes, fossils, and the origin of species

Ever since Darwin there has been a history of debate on the tempo and mode of evolution, Is speciation a gradual process involving the accumulation of minute variations extant within a species, or is it rapid, the results of major organismal reorganization? Does one define a species on the basis of genes, morphology, or geographic or reproductive isolation? In this communi cation I present a model of evolutionary change that is based on the Mendel ian inheritance of mutations in regulatory genes and the fact that most non lethal mutations arise in the recessive state. Since the new recessive alle le will spread through many generations without expression until there is a critical mass of heterozygotes capable of producing homozygotes for the mu tation, the novel feature thus produced will appear abruptly in the populat ion and in more than one individual, This picture of punctuation is consist ent with the fossil record, which typically fails to provide evidence of sm oothly transitional states of morphological change, Given that the first of their kind in the fossil record are organisms in which their novel charact eristic are often more fully expressed or complex than in their descendants , it would seem that, after the mutation involving a regulatory gene is int roduced, the general tendency is for its effects to become diminished, Amon g the implications for speciation is that this process does not depend on e ither reproductive isolation or genetic incompatibility. Rather, barring ef fects on reproductive organs or behavior, homozygotes for a novelty should be able to breed with heterozygotes and homozygotes for the wild state of t he original population, This, in turn, suggests that the species barrier be tween individuals is probably a matter of mate recognition, Anat Rec (NewAn at): 257:15-31, 1999. (C) 1999 Wiley-Liss, Inc.