154 I The Process of Evolution 



integrated genotypes are "winning combinations," and, as demon- 

 strated in the experiments of Mather and Harrison, selection to 

 change them in order to meet a particular environmental stress is 

 countered by selection favoring the retention of the successful inte- 

 grated unit. The unusually sharp break between the two kinds of 

 Maniola jiirtina populations mentioned earlier in this chapter may 

 represent the border between two such highly integrated units. 

 Permanent directional progress is made only when the selective 

 forces operating in favor of change are able to overbalance those 

 operating in favor of retaining the successful combination. There is 

 much to indicate that the phenomenon of overdominance with re- 

 spect to adaptive value (selective advantage of the heterozygote 

 over homozygotes) is one of the fundamental mechanisms contrib- 

 uting to genetic homeostasis in most animals and many plants. How- 

 ever, the term includes all methods of genetic autoregulation of 

 populations. 



GENETIC ASSIMILATION 



When individuals of the plant Achillea lanulosa (Compositae) were 

 transferred from localities at various altitudes in the Sierra Nevada 

 to an experimental garden at sea level, the plants did not all grow to 

 a uniform height (Fig. 7.12). In the now classic experiments of 

 Clausen, Keck, and Hiesey, plants from the higher elevations were 

 much shorter than those from lower elevations. Since all were grown 

 under roughly identical conditions, those from the higher localities 

 were shown to be genetically dwarfed; that is, they had genotypes 

 that tended to produce short individuals regardless of the environ- 

 ment in which they developed. However, when low-altitude plants 

 were divided (giving genetically identical stocks) and these divi- 

 sions were grown at sea level and at mid-altitude, the mid-altitude 

 individuals were shorter than their identical twins at sea level. In 

 other words, the low-altitude genotype interacting with mid-alti- 

 tude environment developed into a plant similar to those with a 

 mid-altitude genotype. Such forms, in which a phenotypic change 

 simulates a genotypic change, have been termed phenocopies. 



There seem to be many situations in nature where such pheno- 

 copying occurs, although rigorous demonstrations of the phenomenon 

 generally are lacking. For instance, many butterflies have spring 

 generations that are smaller and darker than their summer genera- 

 tions, the difference presumably being due to the seasonal variation 

 in the environment. However, in more northern parts of their range, 

 the butterflies have only a single summer generation which is small 



