Homeobox genes, fossils, and the origin of species

Abstract

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 result of major organismal reorganization? Does one define a species on the basis of genes, morphology, or geographic or reproductive isolation? In this communication I present a model of evolutionary change that is based on the Mendelian inheritance of mutations in regulatory genes and the fact that most nonlethal mutations arise in the recessive state. Since the new recessive allele will spread through many generations without expression until there is a critical mass of heterozygotes capable of producing homozygotes for the mutation, the novel feature thus produced will appear abruptly in the population and in more than one individual. This picture of punctuation is consistent with the fossil record, which typically fails to provide evidence of smoothly transitional states of morphological change. Given that the first of their kind in the fossil record are organisms in which their novel characteristics are often more fully expressed or complex than in their descendants, it would seem that, after the mutation involving a regulatory gene is introduced, the general tendency is for its effects to become diminished. Among the implications for speciation is that this process does not depend on either reproductive isolation or genetic incompatibility. Rather, barring effects on reproductive organs or behavior, homozygotes for a novelty should be able to breed with heterozygotes and homozygotes for the wild state of the original population. This, in turn, suggests that the species barrier between individuals is probably a matter of mate recognition. Anat Rec (New Anat): 257:15-31, 1999.

Sudden origins: A general mechanism of evolution based on stress protein concentration and rapid environmental change

“…The “sudden origin” of genetic and phenotypic novelty results in “evolution.” We will not discuss whether “novelty” equates with “species,” but summarize consequences of this model, which, on a more theoretical level, was initially proposed by Schwartz (1999a,1999b). First, one need not invoke a new model of inheritance to accommodate regulatory changes.…”

Progress in understanding hominoid dental development

“…An editorial in Nature (372 : 31-32, 1994) suggested that because of this close relationship is it now possible to ask much sharper questions about the nature and molecular basis of morphological change during human and primate evolution. Weiss (1993) and Schwartz (1999) have recently reviewed many of these ideas and have provided examples of evolutionary changes for which we can postdict the nature of the developmental mechanism involved. Geneticists and developmental biologists have answered many questions about how animals are basically similar and they have defined many shared processes and mechanisms.…”

Interpreting the homeobox: metaphors of gene action and activation in development and evolution

“…One now standard and well‐supported view of homeobox genes is that they are responsible (at least in part) for the evolutionary origin of body plans (Lewis 1994; Raff 1996; Gellon and McGinnis 1998). Paleoanthropologist Jeffery Schwartz has recently taken this a step (or leap) further, invoking the role of homeobox genes in individual development as evidence of evolution by saltation (Schwartz 1999a, 1999b). Neo‐Darwinians focus on microevolution, gradual evolution through the selection of minor adaptations, or what is often expressed as “the survival of the fittest.” But, we might ask, what of “the arrival of the fittest” (Gilbert et al 1996)?…”