SELECTION AND THE RESERVOIR OF VARIABILITY 



occur according to chance fluctuations in numbers and other 

 contributory circumstances. The non-permanent changes, originally 

 of lesser importance, must have come to mask the permanent. 



Now the organism has no means of distinguishing permanent 

 from non-permanent changes in its circumstances. If it is sufficiently 

 flexible genetically to respond to permanent changes, it must also 

 respond to the non-permanent ones. Cyclical changes in the 

 environment may be accommodated by cyclical changes in the 



100 ^ 



fi 80 





CO 



40 



^ 20 



WO 



80 



60 



*» 



IV X 

 t930 



IV X 



If X 

 1933 



/V X 

 /93^ 



IV X 

 /S35 



Fig. 74. — The percentages of the population of the ladybird Adalia punctata with 

 black and red ground colours in April (IV) and October (X) over a period of six 

 years in Buch, Germany. There is a regular seasonal change in the frequencies of 

 these genetically controlled alternatives, red being commoner in spring than it is 

 in autumn (after TimofeefF-Ressovsky, 1940). 



relative frequencies of allelomorphs of genes or super-genes govern- 

 ing polymorphism in an ephemeral (Fig. 74); but this is a special 

 case. Permanent adaptation must generally demand the irreversible 

 fixation of genes, and if this can be brought about by permanent 

 changes, so it can by temporary ones too. In responding thus to 

 non-permanent changes, the stock will gain nothing in fitness, and 

 will lose some of the variability on which prospective adaptation 

 must depend, by the profitless fixation of free variation under the 

 erratic selection. This loss must be made good in the long run by 

 new variability from mutation (Fig. 71), as indeed must any 

 reduction in variability whatever its cause, if the species is to survive. 

 In fact the amount of variability, when at equihbrium in a popula- 

 tion, will be such that the loss, itself proportional to the free and 



295 



