494 INTRODUCTION TO EVOLUTION 



cerebral centers, psychological stimulation (as by bright colors, mating dis- 

 plays and dances) being necessary to induce ovulation. This being so, 

 natural selection would favor development and maintenance of the 

 means of stimulation, no recourse to sexual selection in the Darwinian 

 sense being necessary. This is not to say that sexual selection does not exist 

 or is not important, but only that its range of operation may be more re- 

 stricted than it was formerly thought to be (see Huxley, 1938, for more ex- 

 tensive discussion). 



PULLING THE THREADS TOGETHER 



We now summarize in outline form the main factors 

 and forces making for evolutionary change. 



I. Sources of Variability 



A. New Mutations. These include gene mutations and chromosomal 

 mutations or aberrations (pp. 396-402). Aside from the "instantaneous 

 speciation" possible to plants by polyploidy (pp. 418-420), chemical 

 changes in genes seem on the whole to produce more far-reaching 

 changes than do chromosomal aberrations. 



1. EFFECTS, (a) Structure, (b) physiology, (c) viability, (d) fertility, 

 and other aspects of the organism and its life may be affected by muta- 

 tion. One gene may affect several aspects (be pleiotropic), and con- 

 versely many genes may affect any one aspect. Effects may be small 

 or large. Some of the large effects may be the result of mutations 

 occurring in genes controlling differential growth rates of different 

 parts of the body (pp. 405-418). 



2. MUTATION PRESSURE. New mutations are produced at a continual, 

 though low, rate, thus providing raw materials for evolutionary 

 change. 



B. New Combinations of Genes 



1. MENDELiAN RECOMBINATION. Accumulated genetic variability in- 

 herited from preceding generations constitutes a store of genetic 

 diversity which in bisexual organisms is constantly shuffled and re- 

 shuffled into everchanging combinations (pp. 331-336). 



2. INTROGRESSIVE HYBRIDIZATION. Under somc circumstances one popu- 

 lation may obtain genes from another through hybridization (pp. 

 479-481) when (a) the ranges of the species overlap or (b) migra- 

 tion occurs. 



C. Norm of Reaction or Reaction Range of a genotype. The fact that 

 a genotype confers upon an organism some degree of flexibility in this 

 respect enables the organism to accommodate to various environmental 

 conditions to which it may subsequently attain genetically based adapta- 

 tion (genetic assimilation, pp. 420-424; Baldwin effect, pp. 424-425). 



