The Theory of Population Genetics | 121 



of evolutionary plasticity. The chances of capturing a neighboring 

 higher peak are reduced. 



Figure 6.13c illustrates the consequences of the omnipresent 

 changes in the environment (adaptive peaks becoming valleys and 

 vice versa ) . Here the result depends on the severity of selection and 

 the speed of the environmental change. A species occupying a small 

 field under strong selection pressure may not have the variability to 

 permit it to move to the emergent peaks and may thus be left in a 

 "pit" and become extinct. A population under less stringent selection 

 will merely move as the conditions change. Figure 6.13f/ shows the 

 effects of great reduction in population size and close inbreeding. 

 Random fixation and loss move the population erratically down from 

 its adaptive peak, and inbreeding (producing homogeneity) re- 

 duces the size of the adaptive field occupied by the population. This 

 process (if unchecked) leads ultimately to extinction. In Fig. 6.13e 

 one sees the results of an intermediate relationship between popula- 

 tion size and mutation rate. The population tends to wander from 



Fig. 6.13 I Field of gene combinations occupied by various kinds of 

 populations under different conditions (explanation in text). (From 

 Wright, 1932, Proc. VI Congr. Genetics 1.) 



(a) Increased mutation 

 or reduced selection. 

 ANu, ANs very large 



(6) Increased selection 

 or reduced mutation. 

 4Nu, ANs very large 



(c) Qualitative change 

 of environment. 

 ANu, ANs very large 



(d) Close inbreeding. (e) Slight inbreeding. 



ANu, ANs very small ANu, ANs medium 



(/) Division into local 

 races. ANm medium 



