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John Wiley & Sons, 1999
Reviewed by
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Review
More than one commentator has observed that if there is one thing that Darwin's On the Origin of Species by Means of Natural Selection did not accomplish, it was to address satisfactorily the issue that comprises the first half of its title. To this day we struggle with the question: How do new species arise?
In his new book Sudden Origins: Fossils, Genes, and
the Emergence of Species, Jeffrey Schwartz, a professor of anthropology
at the University of Pittsburgh, has provided an authoritative survey of the ways in which post-Origin of Species evolutionary thinkers have dealt
with this question. He explains how new ideas in genetics were incorporated
earlier in this century, and new ideas in molecular embryology in the
present day, to extend - or in some cases overturn - Darwinian principles.
In the last chapter of Sudden Origins, Schwartz also tackles one of the
most contentious subjects in evolution: To what extent are we to take the abrupt
appearance of new species in the fossil record at face value? Schwartz comes
down firmly on the side of those who conclude that the fossil record more
or less accurately reflects the history of life, and thus cannot be
accounted for solely by gradual evolution through Darwinian natural selection. He
proposes that powerful regulatory factors encoded by the homeobox class of genes have acted to generate novelty in evolving organisms in
a rapid manner.
Schwartz begins the book with four chapters on hominid
evolution. In part, it seems, he does this because it is his area of expertise.
To a certain extent, though, the reason he discusses the important findings in
human evolution and their reception in the world of paleontology and beyond may
be because they support his thesis. Schwartz documents the ways in which
professional paleontologists have consistently attempted to shoehorn newly
discovered hominid fossils into the gaps in the fossil record, declaring them
intermediate forms in a gradual evolutionary progression. His opposing view is
stated succinctly, in the context of
a discussion of limb and digit development: "Because of what we know
about development, we should not expect to find a series of
intermediate fossil forms with decreasingly divergent big toes and, at the same
time, a decreasing number of apelike features and an increasing number of
modern human features."
What is it that we know about development that supports the sudden origin
of new species? Before discussing this topic, which is central to his
thesis, Schwartz presents an extended overview of the contributions of those
19th- and 20th-century scientists who didn't know - or, more
generously, couldn't have known - these things about development. One by one
he describes the work of the intellectual fathers and grandfathers of the
neo-Darwinian evolutionary synthesis. As BioEssays editor Adam Wilkins recently described it, the Modern Synthesis, as it has come to be known, was an attempt, roughly codified in
the 1940s, to incorporate "Mendelian genetics, population genetics, systematics, and paleontology" into
Darwinian natural selection.
The bulk of the book is made up of critical evaluations of the work of these scientists, including Charles Darwin, August Weismann, William Bateson, Thomas Hunt Morgan, Ronald Fisher, Sewall Wright, J.B.S. Haldane, Otto Schindewolf, Richard Goldschmidt, and many others. Schwartz's organizing theme is the century-long debate between those who view speciation as a stepwise, gradual process aided by relatively minor random mutations and barriers to interbreeding, and those who argue that the macroevolutionary process of speciation must somehow be associated with larger, more abrupt changes in animal body plans. His familiarity with the primary literature comes through clearly in these central chapters, which would be a good place to start for anyone searching for a thoughtful introduction to the classic works of evolutionary theory.
Although Schwartz generally seems evenhanded in his evaluations of these
scientists, the nominal heroes of the book are clearly William Bateson and
Richard Goldschmidt. Bateson, who coined the term genetics and whose 1894
work Materials for the Study of Variation is a massive compilation of
examples of discontinuous variation in nature, argued that these
monstrosities constituted strong evidence for rapid evolutionary change.
Goldschmidt, whose 1940 work The Material Basis of Evolution
popularized the phrase hopeful monsters, argued similarly that major organismal
reorganizations could occur in a relatively short period of time (a view
known as saltationism).
Following this historical overview, Schwartz presents his own theory of the sudden origin of species, based on the importance of the homeobox genes in embryonic development. The homeobox genes contain a short motif (the homeobox) that is highly conserved in sequence. When translated into amino acids, this motif allows the proteins that contain it to regulate the expression of other genes. Homeoboxes are present in all animal species examined to date (as well as in yeast and plants), and in the absence of a homeobox gene that is normally present, one or more aspects of development can go horribly wrong. Alternatively, if one of them is expressed at a new location or at a different time, the embryo can develop with novel characteristics that are far from minor.
The most celebrated recent example of such a homeotic mutation discussed by Schwartz is that involving the Drosophila Pax-6 gene. When this gene is expressed in ectopic sites in the fruit fly embryo, extra eyes appear, suggesting that Pax-6 is a master regulatory gene directing eye development. In Schwartz's view, recessive mutations in particular homeobox-containing genes can spread silently through a population until there are enough heterozygotes to allow for the production of animals that carry two mutant alleles, at which time a major change in some aspect of the body plan will suddenly appear.
The homeobox motif was discovered in 1984, and like most observers of this
field, Schwartz has clearly been duly impressed by the power these genes
have in shaping the embryo, a power documented in scores of papers
over the last fifteen years. Still, some have argued that it is possible to
overestimate (or misunderstand) the role of homeobox genes in producing
evolutionary novelty and speciation. After all, a gene like Pax-6 can
be said to be a gene for eye development only in the sense that it is at
the top of a hierarchy of downstream factors that carry out the construction
of an actual eye. This hierarchy, like everything else, must have evolved.
Without it, and without all the other components that are necessary for the
processing of visual information, it is hard to see how
homeotic mutations could lead to viable and better adapted organisms. As
pointed out by Graham Budd in the April 1999 issue of BioEssays, "bithoracic
flies (flies with two pairs of wings) are incapable of flight, not
only because homeotic mutations that generate the extra pair of wings
do not also transform the appropriate musculature, but also because the overall
aerodynamic design of the fly has evolved for one and not two
pairs of wings."
By all means, read Sudden Origins, Jeffrey Schwartz's impressive overview of a century's worth of ideas in evolutionary biology. He has rightly emphasized the importance of homeobox genes in adding some essential molecular and embryological detail to the Modern Synthesis. As for the great debate over the tempo and mode of speciation, however, don't count the gradualists out just yet.
Alan I. Packer is currently a postdoctoral fellow in the Center for Reproductive Sciences and Department of Genetics and Development at the Columbia University College of Physicians and Surgeons.
There is room in evolutionary biology for the investigation of both the roles of natural selection and adaptation and the roles of regulatory gene interaction and expression. But their levels of significance with regard to what is generally referred to as evolution are worlds apart. The former relates to the survival of a species over time, whereas the latter provides insight into the origin of species . . . As such, there is the very real need to return the study of comparative morphology, and especially development, to the fore of evolutionary biology.
You may purchase this book (320 pp.) directly from:
Anthropology Human Origins Website - contains descriptions and images of many of the hominid anatomical features and fossils discussed by Schwartz in the first four chapters of Sudden Origins.
Sewall Wright - a brief profile of the influential evolutionary biologist.
MendelWeb - an educational resource for teachers and students interested in the origins of classical genetics, introductory data analysis, elementary plant science, and the history and literature of science.
Homeobox Page - a site offering up-to-date information about homeobox genes, with particular emphasis on their classification and evolution.
Homeosis and the Tradition Behind This Year's Nobel - an article including background material on the research by three geneticists, Ed Lewis, Christiane Nüsslein-Volhard, and Eric Wieschaus, who won the Prize for their efforts to elucidate the mechanisms that pattern the body during development. By Claudio Stern; from 21stC.
Evolution: Selected Papers and Commentary - an extensive resource providing summaries of evolutionary terms and theories and their historical development, with links to important papers.
