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More Articles on Evolution

Miracles and Molecules

Douglas J. Futuyma

Allen Orr has incisively revealed the profound flaws in Darwin's Black Box. My comments will only expand on a few of Orr's points.

Unfortunately for Michael Behe's argument, molecular biology has only strengthened neo-Darwinian evolutionary theory by providing abundant evidence not only on the history of evolution, but on the mechanisms of evolutionary change. DNA sequences, a rich source of data on phylogenetic relationships among organisms, have in almost all cases confirmed the broad relationships that had earlier been inferred from comparative anatomy. The common origin of all living things has been affirmed by similar DNA sequences, such as those encoding histone proteins in plants, fungi, and animals, and those encoding ribosomal RNA in both those "higher" organisms and bacteria. Although Behe evidently accepts the common ancestry of diverse forms of life, this increasingly indisputable fact greatly narrows the scope for supernatural meddling in life's history.

Behe would find a role for a divine designer in the origin of complex biochemical systems. But molecular evolutionary biology increasingly provides insights into the natural mechanisms by which such systems evolve. One such mechanism, as Orr notes, is duplication of genes, or parts of genes, followed by functional divergence. Gene duplication is the consequence of unequal crossing-over, a process well studied by geneticists, that can both increase and decrease the number of copies of a gene on a chromosome. Such variation in number has been observed, for instance, in human hemoglobins: some individuals have more, and others fewer, than the normal number of hemoglobin genes. (Thalassemias are disorders resulting from such deficiencies.) Over the course of vertebrate evolution, gene duplication has given rise to a family of hemoglobin genes that have diverged in function. The hemoglobin of the lamprey, a primitive jawless vertebrate, consists of a single protein chain (a monomer), encoded by a single gene. In jawed vertebrates such as fishes and mammals, hemoglobin is a tetramer: an aggregate of four chains of two types (alpha and beta), encoded by two genes with related sequences. This tetramer has a cooperative oxygen-binding capacity not available to the lamprey. In salmon, quadruple copies of the beta gene, differing slightly in sequence, yield four types of hemoglobin with different, adaptive oxygen-loading properties.1 In mammals, successive duplications of the beta gene gave rise to the gamma and epsilon chains, which characterize the hemoglobin of the fetus and early embryo respectively, and enhance uptake of oxygen from the mother. Thus a succession of gene duplications, widely spaced through evolutionary time, has led to the "irreducibly complex" system of respiratory proteins in mammals. In addition, some duplicate hemoglobin genes have become pseudogenes: sequences similar to functional hemoglobin genes but bearing mutations that abolish their function. These sequences show that superfluous genes rapidly degenerate.

DNA sequencing has also shown that with slight alteration, or sometimes none at all, gene products acquire very different functions. The crystalline lens of the eyes of vertebrates is composed of one or another protein, such as lactate dehydrogenase, that performs an entirely different, enzymatic function elsewhere in the body. Both lactalbumin, a component of milk, and part of the lactose synthetase enzyme are encoded by genes that differ only slightly in sequence from the gene for lysozyme, which retards infection by breaking down bacterial cell walls, As the Nobel Prize-winning molecular geneticist Francois Jacob said, evolution consists largely of molecular tinkering--producing new objects from old odds and ends.2

Molecular tinkering also includes mixing and matching--combining duplicated pieces of genes into new genes. For instance, about five different modules, in different combinations, compose each of the many proteins involved in blood clotting--and these modules further are constituents of proteins with quite different functions, such as the digestive enzyme trypsin.

Complex biochemical systems then, bear the molecular stamp of their evolutionary origins. Often, these systems can be found, in one or another organism, in a primitive, less complex state--a state that functions adequately, even if not as efficiently as the more complex state that evolved in other lineages. The eye of a mammal is wondrously, perhaps "irreducibly," complex, but an eye without a lens, capable at least of distinguishing light from dark, is better than no eye at all. Likewise, a lamprey's hemoglobin, even if less efficient than that of a jawed vertebrate, suffices to keep lampreys alive. Yet it is doubtful that a mammal could survive with a lamprey-like hemoglobin, for the physiological functions that have evolved in mammals, such as maintaining high body temperature, demand oxygen at a rate that can be supplied only by more efficient, tetrameric hemoglobin. Likewise, it is unlikely that a mammalian fetus could survive without its special hemoglobin. What was once merely an advantage has become a necessity. As Orr emphasizes, irreducible complexity is acquired--it evolves.

Among vertebrates, only a subset--the jawed vertebrates that first evolved about 430 million years ago--have tetrameric hemoglobin, and of these only a subset--the mammals whose ancestors became differentiated from other reptiles about 320 million years ago--have fetal hemoglobin. These facts permit two possible explanations. One--Behe's explanation--is that the common ancestor of all vertebrates, or of all life, was equipped with all the molecular machinery any of its descendants would ever use, and that most of the machinery was lost in most lineages. This hypothesis is not only ludicrous, but also, as Orr points out, makes predictions that are contradicted by evidence. The alternative hypothesis is that new molecular complexities cane into existence in various lineages of organisms at different points in time.

If this is true, and if we were to follow Behe in denying a natural, evolutionary origin of each such instance, then each origin of a divergent, duplicate hemoglobin requires us to postulate a special intervention by the omnipotent designer. Bear in mind that the several new hemoglobins I have described are only a few of the many, slightly different hemoglobins that, like those of the salmon, contribute to the complex, fine-tuned adaptation of diverse organisms to their environments. And these are but a tiny fraction of the "irreducibly complex" molecular adaptations to be found among vertebrates, insects, plants, and other forms of life. Behe, then, must be forced to see the designer's handiwork everywhere. Life must present him with countless instances of supernatural intervention--of miracles.

When scientists invoke miracles, they cease to practice science. Were a geologist to cite plate tectonics, a chemist hydrogen bonds, or a physicist gravity as an instance of the miraculous, he or she would be laughed out of the profession. Moreover, they would not be doing their job, which is to seek answers by posing and testing explanatory hypotheses. Faced with the unknown, as all scientists are, the scientist who invokes a miracle in effect says "this is unknowable" and admits defeat. It is only through confidence that the unknown is knowable that physical scientists have achieved explanation, and that biologists have advanced understanding of heredity, development, and evolution to heights scarcely hoped for just a few decades ago. Yet Behe, claiming a miracle in every molecule, would urge us to admit the defeat of reason, to despair of understanding, to rest content in ignorance. Even as biology daily grows in knowledge and insight, Behe counsels us to just give up.

1 Peter W. Hochachka and George N. Somero, Biochemical Adaptation (Princeton: Princeton University Press, 1984), pp. 279-303.

2 Francois Jacob, "Molecular Tinkering and Evolution," in D. S. Bendall, ed., Evolution from Molecules to Men, (Cambridge: Cambridge University Press, 1983), pp. 131-44.

Originally published in the February/ March 1997 issue of Boston Review

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