78 



The Source of Variability 



PALLIDIPENNIS TUMIDITARSUS PARACHROGASTER TRANQUILLA 



CENTRALIS 



M 



• N ■ 



FULVALINEATA 



5UBTILIS 



ANDINA 



CASTANEA 



Fig. 29. Chromosome morphology in a portion of the genus Diosophila illus- 

 trating the great diversity in organization. (From Patterson and Stone, Evolu- 

 tion in the genus Dwsophila, courtesy of the Macmillan Company.) 



of lethal mutations are the enzyme-deficient mutants of bacteria 

 and molds. Their normal environment lacks the essential substances 

 which the mutants cannot produce; if these compounds were not 

 added to the experimental nutrient solutions, the mutant organisms 

 would never develop. The exact physiological failure causing many 

 mutants to be lethal is not known, but, under proper conditions of 



