"Nothing in Biology Makes Sense Except in the Light of Evolution" is an essay by the evolutionary biologist Theodosius Dobzhansky, criticizing anti-evolution creationism and espousing theistic evolution. The essay was first published in American Biology Teacher in 1973. Dobzhansky first used the title statement, in a slight variation, in a 1964 presidential address to the American Society of Zoologists, "Biology, Molecular and Organismic", to assert the importance of organismic biology in response to the challenge of the rising field of molecular biology. The term "light of evolution"—or sub specie evolutionis—had been used earlier by the Jesuit priest and paleontologist Pierre Teilhard de Chardin and then by the biologist Julian Huxley. As he had said in his earlier presidential address, "If the living world has not arisen from common ancestors by means of an evolutionary process, then the fundamental unity of living things is a hoax and their diversity is a joke." The unity and diversity of life provide central themes for his essay.
Overview Dobzhansky notes that in 1966, Shaikh Abdul Aziz bin Baz espoused the geocentric view that the Sun revolves around the Earth. The sheik said that the heliocentric model was “a mere theory”. Dobzhansky notes that it is a theory with explanatory power, in that it makes sense of things that would be inexplicable otherwise. He goes on to show that the same is true of evolution. Dobzhansky describes the diversity of life: "Between 1.5 and 2 million species of animals and plants have been described and studied; the number yet to be discovered is probably about as great." He describes the adaption of species to niches: "Perhaps the narrowest ecological niche of all is that of a species of the fungus family Laboulbeniaceae, which grows exclusively on the rear portion of the elytra of the beetle Aphenops croneii, which is found only in some limestone caves in southern France. Larvae of the fly Psilopa petrolei develop in seepages of crude oil in California oilfields; as far as is known they occur nowhere else. ... Larvae of the fly Drosophila carcinophila develop only in the nephric grooves beneath the flaps of the third maxilliped of the land crab Gecarcinus ruricola, which is restricted to certain islands in the Caribbean." He asks, "Is there an explanation, to make intelligible to reason this colossal diversity of living beings? Whence came these extraordinary, seemingly whimsical and superfluous creatures, like the fungus Laboulbenia, the beetle Aphenops cronei, the flies Psilopa petrolei and Drosophila carcinophila, and many, many more apparent biologic curiosities? The only explanation that makes sense is that the organic diversity has evolved in response to the diversity of environment on the planet earth. No single species, however perfect and however versatile, could exploit all the opportunities for living. … The evolutionary process tends to fill up the available ecologic niches. It does not do so consciously or deliberately; the relations between evolution and the environment are more subtle and interesting than that.” He writes that such diversity becomes "reasonable and understandable if the Creator has created the living world not by caprice but by evolution propelled by natural selection." He notes that "Natural selection is at once a blind and creative process. Only a creative but blind process could produce, on the one hand, the tremendous biologic success that is the human species and, on the other hand, forms of adpatedness as narrow and as constraining as those of the overspecialized fungus, beetle, and flies mentioned above." He notes that most of the species that have existed over Earth’s tenure are extinct. "All this is understandable in light of evolution theory; but what a senseless operation it would have been, on God’s part, to fabricate a multitude of species ex nihilo and then let most of them die out!" He turns to the unity of life: "From viruses to man, heredity is coded in just two, chemically related substances: DNA and RNA. The genetic code is as simple as it is universal. There are only four genetic 'letters' in DNA: adenine, guanine, thymine, and cytosine. Uracil replaces thymine in RNA. The entire evolutionary development of the living world has taken place not by invention of new 'letters' in the genetic 'alphabet' but by elaboration of ever-new combinations of these letters. Not only is the DNA-RNA genetic code universal, but so is the method of translation of the sequences of the 'letters' in DNA-RNA into sequences of amino acids in proteins. The same 20 amino acids compose countless different proteins in all, or at least in most, organisms. Different amino acids are coded by one to six nucleotide triplets in DNA and RNA. And the biochemical universals extend beyond the genetic code and its translation into proteins: striking uniformities prevail in the cellular metabolism of the most diverse living beings." He notes the similarity of proteins in different species: "the so-called alpha chains of hemoglobin have identical sequences of amino acids in man and the chimpanzee, but they differ in a single amino acid (out of 141) in the gorilla. Alpha chains of human hemoglobin differ from cattle hemoglobin in 17 amino acid substitutions, 18 from horse, 20 from donkey, 25 from rabbit, and 71 from fish (carp)." Another example of the unity of life comes from the molecular sequence of cytochrome C, which Emanuel Margoliash and Walter M. Fitch had shown to be similar in a wide range of species, including monkeys, tuna, kangaroos, and yeast. This unity is further illustrated comparative anatomy and embryology. Either God deliberately arranged things "to mislead sincere seekers of truth" or these similarities are the result of evolution.
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