The Differentiation of Populations 243 



flies show little tendency to hybridize. It is suspected that other un- 

 detected factors also help to keep the two entities apart in nature. 

 In the experiments just described, the two known factors were re- 

 moved by crowding the flies together at low temperatures. In a very 

 short time the action of natural selection established a barrier that 

 was at least partly sexual where one had not existed previously. 



Unfortunately, we know very little about which combinations of 

 gene flow and hybrid inviability lead to fusion of reuniting segre- 

 gates and which lead to total differentiation. This is a wide open 

 field for study. 



Patterns of Differentiation 



The Galapagos Finches and African Cichlids. The complexity 

 of differentiation patterns must not be underestimated. A great 

 many forces seem to have interacted to produce the complicated pat- 

 tern observed today in the Galapagos finches. It is likelv that 

 the Geospizinae are all descended from a small flock (perhaps a 

 single pair) of fringillid ancestors which accidentally reached the 

 islands from the South American mainland. It is highly unlikely 

 that the immigrants would have represented a large and random 

 sample of the parental population; indeed, they were most likely a 

 small and biased sample, containing only a restricted segregate of 

 the parental gene pool. This sampling error means that the selective 

 forces on the islands, even if they were similar to those of the main- 

 land, would operate differentially on the island birds, because they 

 are operating in a genetic environment quite different from that on 

 the mainland. Remember that, in discussing the operation of evolu- 

 tionary forces, a single locus cannot be considered in isolation; the 

 effects of pressure on one locus will depend in part on the composi- 

 tion of the entire genotype. Sampling error and the resultant change 

 in genetic environment that commonly occurs when new colonies are 

 established have been described as the founder principle. It is, of 

 course, a special case of genetic drift. 



Once the finches were established on one island, it was only a 

 matter of time before migrants reached others in the group. Popu- 

 lations on different islands, being subjected to different selection 

 pressures and the effects of the founder principle, probably differ- 

 entiated rapidly. When migrations and remigrations brought differ- 

 entiated forms into contact, selection in many cases must have 

 operated against the tendency to hybridize, as described above. 

 Size and shape of the beak of Galapagos finches are related to the 

 kinds of food eaten and also are used by the birds for identifying 

 mates. Abundance of different food sources varies among the 



