The Source of Variability 



91 



- VERGELANA 



(Mexico) 



Fig. 37. Family tree of the horealis branch of the caddisfly genus 

 Helicopsijche, showing the evolution in the shape of the clasper from a primitive 

 species resembling H. mexicana to the specialized species H. vergelana. 



(Fig. 38). Innumerable examples of similar sequences of small 

 steps in evolutionary change could be cited in both plants and 

 animals. 



If in these cases the intermediate conditions became extinct and 

 only the extremes survived, it might appear improbable for the end 

 results to have arisen from a succession of small changes. So many 

 graded series of this sort occur, however, that the accumulation of 

 small character changes must be extremely common in evolution. 



Large Changes 



Ever since their discovery, investigators have wondered if the 

 homeotic mutants of Drosophila presaged a type of character change 

 important in evolution. These mutants include the one called 

 tetraptera in which the halteres are wing-like, proboscipedia in 

 which certain mouthparts are antenna-like or foot-like; and 

 aristipedia in which the antennae are leg-like. In each case these 

 changes from normal are brought about by some structure coming 

 under the developmental influence of the determiners for another 

 structure. In aristipedia, for example, the cells which would normally 

 become an antenna have come instead under the control of the 



