J. L. BROOKS 117 



classes of a population are somewhat different, and the data 

 given here can be considered normal for these stocks. It can be 

 seen from Fig. 12 that Lake Idsjon has three species of Core- 

 gonus, whereas each of the other two lakes has five, although 

 the graphs for the last two present the data for only four species 

 each. The reason for this discrepancy is that subsequent to the 

 publication of his 1953 paper Svardson discovered (reported in 

 Runnstrom, 1953) that the "blue whitefish" of several Swedish 

 lakes were in reality two closely similar species. Lakes Storsjon 

 and Hornavan are two of the lakes discovered to have two species 

 of "blue whitefish." Populations of these two can be distinguished 

 by their gill raker counts; one has an average of 33-35 (considered 

 34 in Fig. 12) and the other 38-39 (considered 39). 



For populations of closely similar biparental organisms to 

 maintain themselves within the same ecosystem it is necessary 

 ( 1 ) that each be able to find sufficient food in the face of actual 

 or potential competition from the others, and (2) that each be 

 able to maintain its genetic integrity in the face of possible admix- 

 ture with the other population. As the Coregonus populations 

 of these lakes have been able to maintain themselves, it is clear 

 that they must be able to meet the above conditions. Svardson's 

 conclusion that each such population belongs to a species differ- 

 ent from the others with which it continues to coexist is entirely 

 reasonable. 



The manner in which these populations have been able to meet 

 the two conditions for continued coexistence is not known in 

 detail, although we know more of the breeding habits than of 

 the food requirements. We can appreciate the ways in which 



Fig. 12. Characteristics of sympatric Coregonus populations of three 

 Swedish lakes. Common names are used because scientific ones for these 

 populations have not been established. The only certain species criterion 

 is the number of gill rakers. The average number of rakers on the first left 

 gill arch of each population is given in parentheses. The two numbers and 

 the asterisk after the blue whitefish of Storsjon and Hornavan indicate that 

 two very similar species of "blue whitefish" occur in these lakes. Com- 

 parisons of characteristics of same species in different lakes indicate lability 

 of such phenotypic characteristics as size at given age and longevity. Slight 

 differences in average number of gill rakers (in part may be due to sam- 

 pling errors) are in part, apparently, genetically determined. 



