294 



AN INTRODUCTION TO MODERN GENETICS 



promptly move up to the top of it, by a Haldane process. The model is 

 particularly valuable, however, in picturing what happens when no 

 new genes occur. The frequency surface then remains unaltered, and 

 evolution can only occur if the population can be caused to leave its 

 original hill, cross a valley, and ascend to the top of another hill. Wright 

 comes to the conclusion that in a middle-sized population, random 



i:(-^ '■ 



[_=. 



A. Increased Mutation or 6. Increased Selection or C. Qualitative Change of 

 reduced Selection. reduced Mutation. Environment. 



:n r>:\ 





D. Close Inbreeding. £. Slight Inbreedin 



Division into 

 Races. 



Local 



Fig. 128. Diagrams of Evolutionary Processes. — Each diagram represents part 

 of the field of possible gene combinations in a population; the fitness of the 

 combination represented by any point is supposed to be plotted vertically and 

 the resulting surface is indicated by the contour lines, !n each diagram the popula- 

 tion initially occupies the area enclosed by the dotted line, and the diagrams shov/ 

 how various processes shift the population about the field, i.e. change its gene 

 composition. 



(From Wright.) 



fluctuation of gene ratios may be sufficient to carry the population 

 down to the bottom of a valley and on to the slope of another hill, 

 which it then ascends under the influence of selection. In this case the 

 direction in which evolutionary change goes on, i.e. the choice of which 

 hill is ascended, is under the control of chance, though in a long enough 

 period the broadest, and therefore probably the highest, hill (repre- 

 senting the fittest population) is likely to be reached. Over a long 

 enough period, then, the chance element in evolution is less important. 

 These populations of intermediate size, however, only move about 



