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EVOLUTION 



February, 1931 



How Evolution Works 



By H. J. MULLER 



V. X-RAYS CAUSE MUTATIONS 

 TF this general conception of mutation is valid 

 •*• we must regard it as being merely a kind of 

 placing of the problem; we should not yet 

 know just which were ordinarily the critical 

 processes concerned, still less the exact steps in- 

 volved. The conception carries with it, how- 

 ever, suggestions for further experimental in- 

 vestigation. For among the agents of an ultra- 

 microspically random character, that can strike 

 willy nilly through living things causing drastic 

 atomic changes here and passing everything by 

 unaltered there — not a ten thousandth of a 

 millimeter away, there stand preeminently the 

 X- or Y- (gamma) ray and its accomplice, the 

 speeding electron. There is nothing in protoplasm which can 

 effectually stop the passage of X-rays or the related 

 waves of shorter wave-length — gamma and cosmic rays. 

 For the most part, in a cell, the rays will pass through 

 but at isolated, unpredictable spots, depending upon 

 unknown "chance" details of energy-configurations, a definite 

 portion, a "quantum," of the rays will be held up, and part of 

 the energy thus absorbed will issue forth in a hurtling electron, 

 shot out of the atom that stood in the way of the radiation. 

 The atom will be changed thereby, and hence the molecule in 

 which it lies may undergo a change in its chemical composi- 

 tion. But for every atom thus directly changed there are 

 thousands of other atoms changed indirectly. For the electron, 

 shot out hke a bullet (except far faster) , tears its path through 

 thousands of atoms that happen to lie in its way, leaving in its 

 wake a trail of havoc before it is finally stopped. In this 

 process, many of the atoms through which the electron tears 

 have one or more of their own electrons torn out or dislodged 

 from their proper places; this change in the structure of the 

 atoms often causes them to undergo new chemical unions or 

 disunions that in turn alter the composition of the molecules 

 in which the atoms lay. If a gene is a molecule, then, with 

 properties depending upon its chemical composition, it can be 

 shot and altered by the electrons resulting from the absorption 

 of X-rays or rays of shorter wave-length. The only question 

 would be, can enough mutations be caused in this way to be 

 detectable by our present methods, with doses of rays small 

 enough not to kill or sterilize the treated organism? 



With these points in mind, the author undertook in the fall 

 of 1926 a series of experiments designed to test the question 

 at issue. The fruit-fly, Drosophila, was used, since it is so 

 easily and rapidly bred in large numbers and since it rendered 

 possible the employment of special genetic technique for the 

 finding of mutations, that had been elaborated in the course 

 of my previous work on linkage and mutation in this organism. 



It would take us too far afield here to examine this technique 

 in detail. Stocks of flies had been made ap containing in 

 given combinations certain genes with conspicuous effects 

 which would serve to notify the investigator that the chromo- 

 some under consideration was present. On making given 



The opening chapters 

 in our last issue consid- 

 ered the various theories 

 of the cause of evolution, 

 reviewed the principles 

 of genetics, proved the 

 randomness of mutations 

 and traced their origin to 

 the "newly found world 

 of the little," the genes. 

 We'll send this first part 

 to any new subscriber 

 upon request. 



crosses of these stocks with other stocks va- 

 rious combinations of characteristics would be 

 expected in the first and following genera- 

 tions. If flies with some particular expected 

 combination were, however, absent from a 

 given culture, it would mean that a mutation 

 had occurred that had given rise to a lethal 

 gene — one that had killed the flies containing 

 it before they had a chance to hatch. By 

 noting which combinations were missing it 

 could be deduced which chromosome of the 

 fly the lethal was in, and at what place in the 

 chromosome it lay. On the other hand, mu- 

 tant genes having visible instead of lethal 

 effects would be detectable through the ap- 

 pearance of the visible variations, and these too could be traced 

 to their chromosome position through studies of the nature and 

 frequency of the combinations in which they appeared. Mutant 

 genes that were recessive to the normal type, however, and 

 most mutations are recessive, would not have a chance to be 

 seen or found until the second or third generation of offspring, 

 subsequent to their origination. The reason why recessive mu- 

 tations are not evident at once has been explained previously. 



In these experiments the adult flies — in some cases the males, 

 in other cases the females — were placed in gelatin capsules and 

 subjected to doses of X-rays so strong as to produce partial 

 sterility, though the other functions of the flies are not notice- 

 ably disturbed by a dose several times stronger than used here. 

 The treated flies were then bred to untreated mates, and at the 

 same time numerous control matings of the same genetic type 

 were carried on for comparison, consisting of untreated males 

 crossed by untreated females. Thousands of cultures were 

 used in this and subsequent experiments, in order, if possible, 

 to settle the matter beyond any doubt. 



Startling Results 



The results in these experiments were startling and unequiv- 

 ocal. To the toiling pilgrim after plodding through the long 

 and weary deserts of changelessness, here indeed was the Prom- 

 ised Land of Mutations. All types of mutations, large and 

 small, ugly and beautiful, burst upon the gaze. Flies with 

 bulging eyes or with flat or dented eyes; flies with white, purple, 

 yellow or brown eyes or no eyes at all; flies with curly hair, 

 with ruffled hair, with parted hair, with fine and with coarse 

 hair, and bald flies; flies with swollen antennae, or extra an- 

 tennae, or legs in place of antennae; flies with broad wings, 

 with narrow wings, with uprurned wings, with downturned 

 wings, with outstretched wings, with truncated wings, with 

 split wings, with spotted wings, with bloated wings and with 

 virtually no wings at all. Big flies and little ones, dark ones and 

 light ones, active and sluggish ones, fertile and sterile ones, 

 long-lived and short-lived ones. Flies that preferred to stay on 

 the ground, flies that did not care about the light, flies with a 

 mixture of sex characters, flies that were especially sensitive to 

 warm weather. They were a motley throng. What had hap- 



