326 Relations within the Species 



to warrant that designation. Each ecotype is the result of selection 

 by its environment and has become specially adapted for a particular 

 set of conditions. Wide-ranging species are represented in different 

 parts of their ranges by different ecotypes. 



A small population of an ecotype, particularly if it is isolated, will 

 tend to become less variable genetically because of inbreeding; that 

 is, it will come to contain fewer biotypes. With less adaptability 

 the population will be less likely to survive bad conditions and will 

 fail to respond quickly to the occurrence of good conditions. In con- 

 trast, a larger population with more biotypes is more likely to include 

 some individuals that can withstand adverse circumstances and that 

 can take advantage of new variations in the environment. These 

 relations probably account, in part at least, for the fact that certain 

 reduced populations fail to spread widely or to recover a former 

 abundance although environmental conditions appear favorable 

 (Cain, 1944, Ch. 16; and Allee, 1951, Ch. 4). The isolation of seg- 

 ments of a varying population also plays a major role in influencing 

 the course of evolution. This large topic, which is beyond the scope 

 of the present book, is considered in the works referred to above and 

 also by such authors as Elton (1930), Mayr (1942), and Lack 

 (1947). 



An example of the failure of a small population of an ecotype to 

 spread readily is furnished by the distribution of wild irises in Canada 

 (Fig. 9.7). Iris setosa has considerable morphological variation and 

 is distributed widely in western Canada and Alaska. The subspecies 

 7m setosa var. canadensis, however, exhibits high morphological con- 

 stancy and is limited to the Gulf of St. Lawrence region. This sub- 

 species survived the ice age, during which other luxuriant types may 

 have succumbed, but as a relic ecotype it emerged so uniform genet- 

 ically that during the intervening centuries it has been able to repopu- 

 late only the immediate area and is slow in adapting itself to other 

 environments. 



The disadvantage of low numbers may also be illustrated from ex- 

 perience in the oyster fishery. Around the shores of Great Britain, 

 and elsewhere, oyster populations have been greatly reduced by over- 

 fishing. However, after restrictions were placed on the amount of the 

 oystermen's harvest, or in other areas after fishing had been aban- 

 doned entirely because of unprofitable yields, the populations often 

 failed to rebound to their previous large sizes. Gross and Smyth 

 (1946) believe that the explanation is to be found in the lack of 

 genetic flexibility in the small isolated groups of these sessile animals. 



A small population not only may be unable to grow rapidly and to 



