38 AN ANALYSIS OF THE EFFECT OF SELECTION. 



able to the development of a given character can make more likely 

 the occurrence of factorial variations affecting that character, or 

 variations affecting it in a given direction. As a matter of fact, there 

 is no evidence for such a conclusion. The occurrence of mutations is 

 ordinarily such an extremely rare phenomenon that it would be very 

 difficult to obtain statistically significant data in the matter. More- 

 over, when one is selecting for a character, one is examining his animals 

 or plants for that character with unusual care, so that any mutations 

 in that character are very likely to be observed and tested, provided 

 they are in the direction in which selection is being carried out. It 

 follows from these considerations that extremely careful controls are re- 

 quired before any data on these questions can have any significance. 



3. Are variations more likely to occur in the locus of the gene under observation, 

 or in other loci? 



In Drosophila over 25 different and independent mutant factors affect 

 the color of the eye. In mice there are 7 or more independent factors 

 affecting coat-color. According to Little (1915) there are 2 and prob- 

 ably 3 independently segregating factors that affect spotting in these 

 animals. There are at least 14 and probably more definite genes (in 

 different loci) that affect bristle number in Drosophila, not counting 

 the ''modifying factors" studied by MacDowell and the writer. 



In view of these and many similar facts, it is certain that changes 

 in a given character may be brought about by changes in many differ- 

 ent parts of the germ-plasm. If selection of a given mutant race, say 

 hooded rats or Dichset Drosophila, is likely to cause or to isolate muta- 

 tions in the gene that differentiates that race from the normal type 

 (i. e., the hooded factor or the Dichast factor) rather than in any other 

 factors, it follows that mutant allelomorphs must be more variable 

 than "normal" ones. For, by analogy with mice, hooded rats are 

 homozygous for the normal allelomorphs of several possible factors 

 affecting spotting; and Dichset flies are certainly homozygous for the 

 normal allelomorphs of at least 13 mutant factors that affect bristle 

 number. It may be true that mutant factors are on the average more 

 variable than their normal allelomorphs; but no evidence to that 

 effect is at hand; and owing to the great difficulty of statistical treat- 

 ment of the frequency of mutations alluded to above, such evidence 

 will be very difficult to obtain. 1 



In the absence of such evidence, it is more probable that variations 

 will appear in other factors, since there are many of them to vary, 

 but commonly only one that is responsible for the difference under 

 observation. That changes of the one factor itself may occur in selec- 

 tion experiments, however, has been shown by Castle (Castle and 

 Wright, 1916) and the writer (p. 31). It does not follow that selection 

 has caused these variations or that they are more likely to occur than 

 are variations in other factors. 



Evidence has been obtained by Emerson (1917), who used unusually favorable material, 

 that shows clearly that different allelomorphs may at times differ greatly in their mutability. 



