EVOLUTION 321 



that mutations result from minute invisible changes in the 

 chromosomes or genes. 



Efficacy of Selection. — Selection, natural or artificial, of itself 

 cannot bring about an enlargement of variability. In self- 

 fertilized plants both egg and pollen gametes develop in the same 

 parent individual. A parent individual, homozygous for all 

 genetic factors, should presumably produce descendants in which 

 the same genetic factors are reproduced. Johannsen found that 

 when the progeny of single beans of a common self-fertilizable 

 garden variety, of different weights, were raised, pure lines could 

 be sorted out by selection (Fig. 183), each pure line varying 

 about its own mode. A pure line was defined as "the descendants 

 from a single homozygous organism exclusively propagating by 

 self-fertilization." Selection within a pure line failed to shift 

 the mode of the variation curve in either a plus or a minus direc- 

 tion. The only result of selection in the general population was 

 the isolation of pure lines, which were beans of similar germinal 

 constitution. The difference between pure lines rests upon a 

 hereditary basis; the variations within pure lines upon non- 

 heritable, environmental factors. Selection within a pure line 

 failed to produce larger or smaller beans because such size 

 differences were the result of environmental factors. New 

 types of variants within a pure line can only be produced by 

 mutation. 



The term phenotype, first used by Johannsen, is now generally 

 applied to a group of individuals having similar external features. 

 A genotype includes individuals having the same germinal com- 

 position. Put in another way, the genotype refers to the funda- 

 mental genetic constitution of an organism, including all the 

 genes; while the phenotype includes all of the characters appear- 

 ing in the individual, regardless of genes, which though present, 

 do not produce visible effects. The general population of beans, 

 or the phenotype, is made up of a number of genotypes. In an 

 ordinary monohybrid Mendelian cross involving a dominant and 

 a recessive character, the 3 :1 ratio of individuals in the F% is com- 

 posed of two phenotypes, one showing the dominant trait and the 

 other the recessive. The dominant individuals (comprising a 

 single phenotype) consist of two genotypes, one homozygous and 

 the other heterozygous. The recessives are homozygous and of 

 the same genotype. 



