THE EFFECT OF LINKAGE 



Balanced genotypes of intermediate phenotype are favoured ; but 

 owing to the flow of variability, which arises from the outbreeding, 

 the extreme types can never be lost. They will constantly reappear 

 by segregation. Thus the population must always vary around the 

 optimum, and fitness can never be so high as in an inbreeding group. 

 The flow of variability ensures, however, that should the environ- 

 ment change, and thereby shift the optimum phenotype, the 

 population will be able to achieve a corresponding adjustment by 

 utilization of the free variability which is always available, 

 hidividuals approximating to the new optimum will be present and, 

 being favoured by the new conditions, will contribute more to the 

 next generation. Thus the genetical constitution of the population 

 can, and will, change to meet the demands of the new conditions. 

 The system does not maintain the maximum immediate fitness, 

 but it is flexible. As compared with inbreeding it is certainly 

 at a disadvantage for the moment; but it confers on an organism 

 a much greater prospect of leaving descendants in a changing 

 world. 



The Effect Oj Linkage 



The disadvantage of high variability under the outbreeding 

 system may be mitigated, and the opposing needs of fitness and 

 flexibihty be partly reconciled, in a way which has been revealed 

 by selection experiments. One of these, described by Sismanidis, 

 was concerned with the number of bristles borne on the scutellum 

 of Drosophila melanogaster. This number is almost always 4, but in 

 some mass-bred stocks occasional flies, mainly females, are found 

 with 5. Commencing with such females mated to normal males, 

 Sismanidis endeavoured to raise the average bristle number by 

 selective breeding, hi this he was successful. Two of his selection 

 lines, maintained by brother-sister mating, are shown in Fig. 73. 

 After 23 generations of selection the average was raised in females 

 from below 4-1 to over 5 '2, but the advance had not been smooth. 

 There was a large change from just below 4*2 to over 4*5 between 

 generations 2 and 3 in one line, and between generations 6 and 7 

 in the other. In the next 1 1 generations in the one and 7 in the other 

 the advance was only o*i, but between generations 14 and 17 a 



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