Page six 



EVOLUTION 



June, 1931 



How Evolution Work 



By H. J. MULLER 

 Professor of Genetics, University of Texas 



VIII. Multiplication and Selection: 

 Turning Accident into Order 



TT does, at first sight, seem incredible 

 •^ that all the marvelous organizations in 

 the living things about us could have been 

 put together by anything partaking of the 

 nature of accident. But we must remember 

 that it did not fall together all at once, and 

 that it was all made possible by that ahnost 

 magical property which life owes to the 

 gene — the power of multiplication of mu- 

 tated individuals. 



For many millions of years, blind chemi- 

 cal forces must have acted and interacted in 

 early times to build up ever different and 

 more complicated organic compounds and systems of com- 

 pounds. A turning-point was reached when from these shift- 

 ing combinations those self-multiplying yet mutable materials 

 which we call genes happened to become formed. From that 

 time on the different genes, or the little systems of organic 

 matter containing an association of genes, would necessarily 

 enter into a destructive competition for multiplication against 

 each other, until, step by step, through mutation, or the altera- 

 tion of the gene, and heredity, or the multiplication of the 

 gene, the complicated present day life became differentiated. 



It will be worth our while now to examine more closely just 

 how it is really the peculiar power of multiplication of mutant 

 forms which turns this trick of converting accident into order, 

 by making such very extraordinary combinations of accidents 

 possible as could not otherwise occur. For some reason, this 

 fundamental feature of the matter does not seem to have been 

 fully realized. 



In examining the process of evolution, let us be content at 

 first to make our case a very simple one, and to proceed for a 

 while in a very elementary fashion, in order to avoid con- 

 fusion. Let us first see how just a simple combination of ad- 

 vantageous changes, or mutations, may be obtained in an 

 organism. Suppose we start 

 with some extremely simple or- 

 ganism, represented by the 

 straight vertical line at the top 



of Diagram I. We will now --;;r: 



allow it to reproduce, and allow 

 enough time to elapse so that 

 some mutations or other will 

 have appeared in each of its 

 descendants (they need not be 

 regarded as first-generation off- 

 spring) . In our diagram these 

 descendants are shown as ver- 

 tical lines placed in a horizontal 

 row just below the vertical line 

 representing the ancestral indi- 



This article concludes the 

 series. Previous chapters re- 

 viewed the various theories 

 of the causes of evolution, 

 proved the randomness of 

 mutations, traced their ori- 

 gin to the genes, and told 

 of X-ray experiments that 

 increased the number of 

 mutations 15,000 per cent, 

 indicating short wave radia- 

 tion as a natural cause of 

 evolution. We'll send these 

 chapters to any new sub- 

 scriber upon request. 





Diagram 1. 



How Multiplication allows the origination of a beneficial 

 combination of variations (gi g2) 



/idual, their derivation from which is indi- 

 :ated by dotted connecting lines. We may 

 suppose that multiplication has brought 

 ibout the existence of seven of these des- 

 cendants, each with a different "chance" 

 mutation, indicated by a differently shaped 

 spot, and lettered from aj to g^. g^ may be 

 taken to represent the "good" mutation — • 

 the one of an advantageous nature, which 

 is in the path of progress, that happened to 

 occur amongst all the others of a disadvan- 

 tageous or neutral kind. Now allow a similar 

 length of time to elapse again, in which mul- 

 tiplication and chance mutations take place 

 much as before. The individual with the 

 "good" mutation, gi, thus multiplies to 

 form seven again, each carrying gj (i.e., the multiplication has 

 involved the variation itself), but, in addition to gi, each of 

 the individuals carrying it now carries a second mutation, 

 lettered from ao to g2. Among these second mutations we may 

 again suppose that only one of the seven, g2, is "good," in the 

 comLination in which it occurs. Thus we get a combination, 

 in one individual, of two good mutations, gi and g2, which 

 supposedly have properties that "fit well together," interacting 

 so as to work out advantageously in combination. 



Some or all of the other individuals of the previous genera- 

 tion, bearing mutations ai to fi, may also have multiplied. 

 Whether or not they did would not affect our desired result — 

 the attainment of the gj go combination — at all, provided only 

 that the gi individual itself had been able to multiply and 

 mutate as indicated. If all the individuals of the previous 

 generation had multiplied to just the same extent as the one 

 having gj did, there would obviously have been 7 x 7 or 7^, ot 

 49, individuals formed bearing some combination of mutations, 

 and of these forty-nine different combinations just one would 

 be the "good" combination — gj g^- Accordingly, without any 

 "natural selection," there would be one individual in forty-nine 

 having the "good" combination. In still other words, the 



"chance" of the good combina- 



Ition being present in any par- 

 ticular one of these final indi- 

 ._.^-,-._-.v:-» ;?-;.'■ : viduals, in the absence of natu- 



.z-'-ZZ''-''',--'' ,■'' ' r^l selection, would have been 



1/49. It is to be further <h- 

 served that, no matter how few 

 or how many of the above forty- 

 nine individuals were actuallw, 

 produced, the ancestors of the 

 gi gi individual (namely, the 

 gi -bearing individual and its 

 predecessor, the simple line) 

 had multiplied at the rate re- 

 quired for doing their share in 

 the production of these hypo- 







