Sudden Origins: Fossils, Genes, and the Emergence of Species

Jeffrey H. Schwartz Wiley: 1999. 408pp £19.99, $27.95)

Evolution by radical mutation: “Tragic anatomies” by Jake and Dinos Chapman. Credit: SAATCHI GALLERY, LONDON

The origin of species has long fascinated biologists. Although Darwin's major work bears it as a title, it does not provide a solution to the problem. Does Jeffrey Schwartz give one? I am afraid that, in general, he does not, but the book is still interesting.

Schwartz presents a detailed and informative historical account of evolutionary biology. In fact, the book could be read as a history of evolution, and will probably occupy such a niche. Schwartz is much more ambitious than this, however: he wants to convince the reader that his “new evolution” is the Holy Grail of the field.

These are the elements of his proposal. First, forces of speciation are largely independent of selection within the species. Second, speciation is typically triggered by a mutation in a homeobox gene. Third, such a mutation is likely to become fixed in population isolates. Fourth, the mutation remains unnoticed for a long time because of its recessive nature. Fifth, when it reaches homozygosity in the small isolate, it will appear in several individuals simultaneously. Sixth, the mutation, since it affects development, is likely to affect mate choice as well; hence, mutant homozygotes are likely to mate among themselves.

Although Schwartz has in one sense resurrected Richard Goldschmidt's “hopeful monsters” (organisms with radically altered characteristics, produced in single large steps), he thinks that he has managed to avoid the associated pitfalls. The homeobox mutations would be macromutations in their effects, but from the genetic point of view they would qualify as micromutations, since no large genomic rearrangements are implied. The problems of finding a mate also seem to be solved by the dynamics of recessives (mutant homozygotes are thought to appear simultaneously, following a lag phase of heterozygosity). And, since homozygosity and preferential mating (pre-zygotic isolation) go hand in hand, there would be no transitional forms.

A drawback of this scenario is the absence of a population-genetic treatment. One is therefore left uncertain about the effects of population size, fixation time, and so on. It is not even certain whether Schwartz thinks that the mutant homozygotes have a selective advantage or are effectively neutral for the individual. In the latter case, the logic of the scenario still suggests that they would influence the dynamics of sexual selection, an interesting possibility. There is nothing wrong with dreaming up a verbal evolutionary scenario, but these days this surely cannot be the last word.

Another shortcoming of the book is the lack of at least one documented case of speciation that may have happened following Schwartz's scenario. Examples that could be taken as evidence for the component processes are discussed in some detail, but this does not rule out the possibility that one case that fits the complete scenario would never be found.

Schwartz shares a distrust of selection with some contemporary biologists. This is, I think, why he chose to ignore, for example, Peter Sheldon's work on the gradualism apparent in the evolution of the pygidial ribs of trilobites. When he discusses the celebrated computer study by Dan Nilsson and Susanne Pelger on the evolution of the optical structure of a fish eye, he mistakenly suggests that the intermediates are not selective improvements on the previous forms. It is revealing that he dismisses this scenario by saying : “Do we actually need to invoke such an elaborate thought experiment in order to understand the origin of the vertebrate eye, or any eye, for that matter? I think not. And the reasons lie in knowing that there are homeobox genes for eye formation and that when one of them, the Rx gene in particular, is activated in the right place and at the right time, an individual has an eye.”

This is utterly misleading. Schwartz ignores the fact that homeobox genes are selector genes. They can do nothing if the genes regulated by them are not there. It is these genes that specify in detail the adaptive structure of the organs. To be sure, turning on a homeobox gene at the wrong place can result in the appearance of an ectopic organ, but only if the genes for that organ are present in the same individual. It is totally wrong to imply that an eye could be produced by a macromutation when no eye was ever present in the lineage before. Homeotic mutations that reshuffle parts do happen, and sometimes they may have led to fixation of real evolutionary novelties, but this does not mean that such changes are implied in the majority of speciations. In fact, macromutations of this sort are probably frequently maladaptive, in contrast to the vast number of past and present species — not to mention the fact that morphological differences between related species can be minute.

The history given in the book is fascinating, and some of the suggestions merit further work, but the reader is likely to be deeply disappointed more than once before the end.