130 THE SOCIAL LIFE OF ANIMALS 



of Mount Major Adaptation, and if selection con- 

 tinues may be expected to move up that adaptive 

 peak. A continually changing environment is un- 

 doubtedly an important factor in evolution. 



The effects of population size are illustrated in the 

 next three diagrams. The general background is the 

 same as in Figures A and B. In Figure D is shown 

 the effect of a decided reduction in population size, 

 and consequently in variability, in the species that 

 formerly occupied Mount Minor Adaptation. It is 

 in fact so small that selection has become ineffective 

 and the different hereditary qualities shift to chance 

 fixations. As non-adaptive characters become fixed at 

 random the species moves down from its peak over 

 an erratic, unpredictable path. With reduction of 

 population size below a certain minimum, control by 

 selection between genes disappears to such an extent 

 that the end can only be extinction. 



With the species population intermediate in size, 

 with the same mutation and selection rates as before, 

 gene frequencies move about at random but without 

 reaching the degree of fixation found in the preced- 

 ing case. Since it will be easier to escape from low 

 adaptive peaks, the population will tend finally to 

 occupy the more adapted levels. The rate of progress 

 is, however, extremely slow. 



Finally, in Figure F, we see the case of a large 



