The Differentiation of Populations 245 



The food requirements of the various isolates may have been too 

 similar, or the number of available niches too restricted to permit 

 the sort of habitat specialization seen in Darwin's finches. 



The result is that each form now occupies the areas to which it is 

 best suited. The historical details will probably never be known. 

 Some forms, perhaps, lacking the genetic variability to differentiate 

 further, became extinct. This could happen when severe conditions 

 in an area greatly reduced the food supply, aijd larvae of another 

 Erebia species proved to be much more efficient at utilizing the 

 restricted food supply. Some forms (e.g., E. calcarius and western 

 E. cassioides) may never have met. It is interesting to note that, in 

 the E. tijndarus group, hybrid sterility exists in crosses between dis- 

 tant relatives such as E. iranica (n = 51) and E. calcarius (n — 8) 

 where there is no behavioral isolation. These forms have apparently 

 never been in contact; thus the selective basis for the development 

 of isolating mechanisms has never been present. 



Differentiation of Parasites. Patterns of differentiation that in- 

 volve strains preferring different foods or hosts are poorly under- 

 stood. It has frequently been observed that, in the laboratory, strains 

 of parasitic organisms may be successfully switched from the usual 

 host organism to another by transfer of large numbers. For instance, 

 the human louse, Pediculus hiimanus, can be converted into a rabbit 

 louse. Large numbers are transferred, and the relatively few able 

 to feed survive and reproduce. This process of selection eventually 

 results in a strain that is happy on rabbits but unenthusiastic about 

 men. (Individuals will feed on men, but a colony will not thrive.) A 

 similar selective process may well be responsible for the transforma- 

 tion of head lice into body lice when the former are subjected to the 

 normal environment of the latter. (It has been suggested that the 

 genetic structure of louse populations encourages a plasticity that 

 makes the transformation in either direction relatively simple.) 

 Needless to say, this structure is itself undoubtedly a result of selec- 

 tion. The Ascaris may differ from the Pediculus principally in that 

 the Ascari are not so protean genetically. 



There can be little doubt about the selective advantage accruing 

 to those cuckoos whose eggs closely match those of the host bird. 

 Rates of desertion of eggs laid in the nests of the usual host are 

 much lower than of those laid in the nests of unusual hosts. How- 

 ever, the exact way in which gentes developed and are maintained 

 is still somewhat of a mystery. It seems unlikely that gentes are 

 genetically isolated from each other. There is no sign of differentia- 

 tion among them except in the egg habitus; such differentiation 

 would be expected if each gens was an isolated exolutionary unit. 

 However, because only superficial characteristics are studied in many 



