C. L. PROSSER 345 



tions. Reproductive isolation may result from chromosomal 

 arrangement or mutation, from gross morphological, physiolog- 

 ical, or psychological incompatibility. Speciation is now effec- 

 tively complete. 



5. Further changes resulting in habitat or ecological niche 

 selection may occur so that if the two species come to overlap 

 in range they may be separated ecologically as well as repro- 

 ductively. The initial or primary adaptive difference may have 

 lost its effectiveness. 



Physiological variation is important at steps 1, 4, and 5. 



Fig. 2. Schematic representation of distribution of a character in two 

 genotypes Gj and Go and in two populations of each genotype indicated 

 by phenotypes Pi and P 2 for G t and P', and P' 2 for G 2i 



Figure 2 indicates two similar genotypes, each limiting the 

 range within which phenotypic variation can occur with respect 

 to some specified character. Two populations of the same geno- 

 type ( Gi ) living in two different environments, for example at 

 two latitudes, may show phenotypic patterns indicated by Pi 

 and P2. By acclimation, individuals of one of these populations 

 can shift their phenotypic expressions to correspond to those of 

 the other population. For example, the minimum lethal tempera- 

 tures can be altered for cold and warm water populations of 

 fish. In the other genotype or race (G2), one population P'i is 

 similar to Pi- in respect to the measured character, but by accli- 

 mation it is found that P'i cannot be converted to Pi but that it 

 can go beyond either Pi or Pj toward a new type P'^. According 



