SELECTION AND VARlAlilLITY 



Others. Again wc see that, although a change can be achieved, there 

 are obstacles to be overcome first. The very linkage v^hich is so 

 powerful in reconciling the needs of flexibility and fitness when 

 displayed between genes of similar action, results in an inertia when 

 displayed between genes of divergent action. 



The extent of this inertia will be dependent on the degree of 

 selective disadvantage resulting from the correlated response as 

 compared with advantage arising from the primary response. It 

 the correlated change should occur in a character of minor impor- 

 tance to the organism's fitness, it could result in the fixation of a 

 new expression of this character under the action of an unrelated 

 selective force, hi other words, subordinate characters may be pushed 

 about by the selective forces acting on capital characters. Selection 

 can in this way produce a change from which no selective advantages 

 could arise, and which therefore will show no trace of the agency 

 which caused it. 



Subordinate selection is seen where the character in question was 

 once of great moment for the organism but has lost all its importance. 

 Such a character, for example, is sight in cave animals. Once the 

 selective advantage of adequate sight is removed by the adoption 

 of cave life, selection for other characters, such as sense of touch, is 

 able, through correlated responses, to bring about the breakdown 

 and atrophy of the visual mechanism which has been degraded to the 

 rank of a subordinate character. 



We are now in a position to look back at the situation as it w^as 

 left by Darwin. To Darwin, variation was what he saw in the 

 members of a living species. He discovered the great problems of 

 inbreeding and outbreeding, of adaptation and selection; but, in 

 terms of the variation that he saw, these problems could not be 

 solved. Now we realise that underlying the visible variation arc 

 organisations of genes and chromosomes which exist largely as 

 means of suppressing the appearance of variation. They contain the 

 variability and reveal it in ways which the principles of Mendclism 

 and Morganism enable us to understand. The operation of these 

 principles is, of course, complex and in a sense abstract. But it is 

 capable at every stage of being submitted to experimental test, and 

 of yielding the predictions that are necessary both to the theory ot 

 evolution and the practice of plant and animal breeding. 



300 



