244 I The Process of Evolution 



islands, and it is likely that selection caused some differentiation 

 in the beaks and associated structures in isolation. When contact 

 was reestablished, selection probably caused greater differentiation 

 of these structures because they were important in recognition. 

 Such differentiation also seems to have an additional advantage in 

 reducing the types of individuals eating the same kind of food. Such 

 selection for "reduced competition" obviously occurs, but its exact 

 mode of operation is unclear. 



The history of the Haplochromis swarm must have been similar 

 to that of the Galapagos finches. Migrants from river systems col- 

 onized ancient Lake Victoria. Multiple colonizations (separated by 

 appropriate time intervals) could alone account for the observed 

 diversity. New arrivals found old immigrants already partially dif- 

 ferentiated; selection against hybridization finished the job. In 

 addition, repeated cycles of drying and inundation may have al- 

 ternately fragmented and reunited segregates, permitting the mech- 

 anisms of differentiation to act. It is also possible that areas of 

 different types of bottom, shoreline, water depth, etc., acted (and 

 act today) as intrinsic barriers to the dispersal of the various forms 

 within the lake. 



Sibling Butterfly Species. The Erebia tijndarus group is another 

 excellent example of the subtle interactions possible between differ- 

 entiated forms. For instance, in spite of their different life-cycle 

 adjustment to altitude, E. cassioides and E. nivalis seem to be 

 strongly influenced by each other's presence or absence, the alti- 

 tudinal restriction appearing where the two forms occur together. 

 The present distributional picture seems to represent the results of 

 differentiated populations interacting over a varied field. As with 

 Darwin's finches, the exact nature of these interactions is difficult 

 to specify. "Selection to avoid competition" is not an explanation 

 in itself. It is necessary to know exactly how differential reproduc- 

 tion of genotypes came about. 



Presumably there was some differentiation before meeting. For 

 instance, when the populations ancestral to today's E. nivalis and 

 E. cassioides populations came together, the E. nivalis may have, on 

 an average, lived more toward the upper limit of the joint range 

 and E. cassioides, on an average, nearer the lower limit. Then there 

 must have been some advantage accruing to the E. nivalis genotypes 

 that "preferred" the high altitude location and those E. cassioides 

 that tended to remain low. Perhaps the relative scarcity of the other 

 form reduced the possibility of wasting gametes through unsuccess- 

 ful hybridization. Then again, maybe unlike larvae tended to can- 

 nibalize each other. Perhaps waste products of the E. nivalis larvae 

 tended to inhibit the growth of E. cassioides, and/or vice versa. 



