GENE MUTATION 



prospective fate is elimination by natural selection. Yet a priori one would 

 expect that there must be a limited number of ways in which a particular 

 gene can mutate. For any known chemical substance, there are certain 

 classes of reaction which are possible and certain classes which are not 

 possible. Whatever theory of the gene one may hold, the genetic material 

 must have definite physical and chemical properties which will restrict 

 the potential scope of mutation. That this is actually the case is indicated 

 by the frequency with which many mutations recur, for example albinism, 

 which is common in most groups of vertebrates. 



Experimental Production of Mutation. As early as 1927, H. J. Muller 

 showed that mutations were produced in Drosophila at several hundred 

 times the normal frequency if the gonads were X-rayed. He selected sex- 

 linked lethal mutations for special study because of the ease with which 

 they can be identified. It has since been shown by Muller and others that 

 any high-energy radiation will produce mutations. In 1947, Muller was 

 awarded the Nobel Prize in Medicine on the basis of this work. With any 

 type of radiation, the rate of induced mutation is directly proportional to 

 the dosage, but is independent of the rate of administration of the radia- 

 tion. Chemical mutagens were sought unsuccessfully for many years, but 

 during World War II, Charlotte Auerbach demonstrated that mustard gas 

 is as effective as radiation. Subsequently, mutagenic activity has been 

 found in such diverse substances as urethane, formaldehyde, peroxides, 

 manganese chloride, aluminum chloride, purines, and pyrimidines, and 

 the list may well become indefinitely large. 



When Muller's work was first reported, the question was naturally 

 raised, is cosmic radiation, or some other naturally occurring radiation, 

 responsible for mutation in nature? Turel presented evidence that the 

 mutation rate is higher in alpine areas than in lowlands, and he suggested 

 that this might be caused by ultraviolet radiation from the sun, which is 

 known to be more intense at higher altitudes. But it is also possible that 

 the great diversity of alpine plants has been caused by selection of difiFer- 

 ent mutants occurring in isolated areas, without the need of a high muta- 

 tion rate. Babcock tried raising Drosophila in areas of high and low 

 natural radiation. Significant differences in mutation rate were not ob- 

 served. Hence it must be admitted that studies of experimental mutation 

 have not yet led to the understanding of naturally occurring mutation. 

 Chemical mutagens may well be of great importance here. 



GEOGRAPHIC SUBSPECIES AND 

 NEO-DARWINIAN EVOLUTION 



An important aspect of natural species is the fact that the various types of 

 variability occurring within a species are not scattered evenly over its 

 entire range, but rather that local populations, commonly more or less 

 isolated from their neighbors, show distinctive patterns of the variable 

 characters of their species, with the result that they may be defined as 

 subspecies. They are fertile in crosses, and intergrades between adjacent 

 subspecies are commonly found wherever their ranges meet. But the 



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