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Though the book is touted as presenting a revolutionary new theory, it is more about the history of debate on the tempo and mode of evolution. The amount of literature Schwartz surveys is impressive. Pushing aside saltationists Bateson, de Vries, Goldschmidt and Schindewolf; Mayr, Dobzhansky and Simpson formulate the gradual "synthetic theory" of evolution. One can easily imagine Mayr as saying "Don't bother me with the data, I have my theory." Schwartz makes the case well, make no mistake about it, evolution saltates. It is not gradual. According to Schwartz, the role of natural selection is in refining a species, not in the creation of species. In transition, Schwartz notes that there is a growing trend with in evolutionary biology to forget about the gradual accretion of minute changes and look to when and where homeobox genes are turn on or off. Finally, in the last 20 pages of his book, Schwartz presents his theory: A mutation occurs in a homeobox gene. The mutation is recessive so that is not expressed. The mutation is inherited and spreads throughout the population without being expressed until they are so numerous that two mates often find each other that are heterozygous for this mutation. One quarter of the offspring then have two recessive alleles and the trait is expressed. The phenotype of these individuals is so different that they prefer each other for mating. Then, somehow, the mutated gene is converted to the dominant state.
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Critique of the Theory |
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A recessive mutation permeating a population without being expressed until heterozygotes for that allele mate and produce offspring that have the new phenotype seems like a usful concept to me. For this I give Schwartz credit, though it is hardly a breakthrough, and I would not be surprised if I found the idea mentioned a century ago. The mate recognition species barrier also seems like a good concept. The good about the theory being said, I have a bevy of crticisms. Schwartz seems to be suggesting that a simple homeotic mutation produced eyes, and that a simple homeotic mutation produced teeth, and that a simple homeotic mutation produced limbs. I have a problem with this. Though new limbs require no new cells types to be invented, teeth require the services of a gene that produces enamel at the right time and in the right amount. And I imagine that there are specialized genes activated in the formation of an eye. In any case many genes must be active at specific coordinated times to produce a new structure. An obvious morphological feature may appear from a single mutation, but not an entire useful structure. See page 369 where he states that a mutation in one of these regulatory genes can result in novel features "that emerge in a full-blown and viable state.'' Schwartz suggests that after the mutated gene permeates the population in the unexpressed heterozygotic state, that it eventually converts somehow so that it is no longer recessive, but dominant. How does a recessive allele convert to the dominant state? Schwartz doesn't say. What about an example of such a conversion? Schwartz doesn't give any. What about the effect of multiple alleles? Schwartz doesn't mention this. Incomplete dominance? Not mentioned. Homeobox genes have multiple roles, and I wonder if Schwartz is aware of this. Schwartz never attempts to explain what are the biochemical processes that make a gene dominant or recessive. It is easy to imagine why some genes are dominant. For example, there is a gene that causes brown eye pigment in the iris. A gene mutation can make the protein product have no effect at all. A person with two of these defective alleles will have blue eyes. But homeobox genes produce transcription factors and bind to DNA with the cooperation of other proteins. Now how do the terms "dominant" and "recessive" apply? I wish Schwartz would spend some time on this. I find it neglectful to propose a theory based on recently discovered developmental genes and then to use Mendelian concepts of dominant and recessive alleles. At any rate, the conversion to dominance is not essential to his theory, since the new homozygous recessive population can simply not mate with members of the old population with the old dominant gene. It would appear that Schwartz has not as much as read an undergraduate textbook on genetics.
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This book should not have been published with this flawed theory. Nevertheless, I recommend it for several reasons: Schwartz demonstrates in lucid detail how the synthetic theory of evolution (neo-darwinism) came to be by ignoring pertinent data. Those who ignore data do not make discoveries. Schwartz sees the data. Schwartz recognizes the need for a new theory. Many people have been deluded into believing that a new theory of evolution is not needed. Schwartz looks in the right place for a new theory: in developmental genes. Multi-cellular organisms follow a program during development instructing cells on how many times to divide, when to differentiate into specific cell types, and when to die. No one knows how this program works or where it is encoded. The alteration of this program is a part of that which makes new species. The neo-darwinian concept that the natural selection of micro-mutations can be extrapolated to explain macro-evolution is dead in the water. |
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