Species and Species Change 



141 



Chimo 



nunglello 



Tribol 

 Ancestor 



Fig. 58. Family tree of the known species of the caddisfly tribe Philopo- 

 tamini. All the surviving lines are cool adapted with the exception of the two 

 leading to Chimarra and Gunungiella which gave rise to warm adapted forms. 

 A indicates three species known from Baltic amber. 



tions. In such a situation at least some selection factors would 

 differ on each segment of the periphery of the range. At any one 

 time the species would therefore contain a sort of internal tug of 

 war, the selection factors on one periphery tending to pull the 

 genetic mode in that direction, the selection factors on the opposite 

 periphery tending to pull the mode in that direction. 



Species in general exhibit pronounced genetic homeostasis, which 

 causes, essentially, an inertia to genetic change. This homeostatic 

 condition is a balanced heterozygosity buffered by modifying genes 

 or other elements so that essentially the same phenotype is pro- 

 duced by a wide range of genotypes ( Lerner, 1954 ) . After a species 

 had been in the same environment for a long time, presumably it 

 would have reached a homeostatic condition in which the genetic 

 extremes of the edges had been molded by polygenic action and 

 buffering into a well-coordinated meshing of physiological reac- 

 tions. Small mutations would be damped and absorbed by the 

 buffering action or polygenic complexities and have no perceptible 

 phenotypic effect. 



To initiate adaptive shifts in a phylogenetic line would require 

 changes sufficient to counteract the damping effect of genetic home- 



