EVOLUTIONARY MECHANISMS 



301 



colouration has a real value in nature, and are thus evidence that 

 natural selection is a real phenomenon. 



Fig. 129. Selective Advantages of Some Genes in Drosophila funebris. — ^The 

 influence of selection was measured by making up a cross in which a 1 : 1 ratio 

 was expected, and determining the deviation from this ratio. The selection is 

 influenced by temperature, crowding and the genotypic milieu. Note that ev 

 has some positive advantage over the wild type. The selective advantages are 

 given as percentages compared to wild type. (Data of Timofeeff-Ressovsky.) 



It is common to find that different genes have different selective 

 values under laboratory conditions. Timofeeff-Ressovsky^ in particular 

 has studied the relative viabilities of different genes in Drosophila, by 

 ^ Timofeeff-Ressovsky 1934a. 



