242 I The Process of Evolution 



Most species of plants seem to be strongly restricted ecologically. 

 Where the ecological barriers are strong, genetic interchange does 

 not often take place, and the hybrids with their intermediate eco- 

 logical preferences do not survive. But when the ecological barriers 

 break down, as in habitats subject to erosion or disturbance by man 

 or glaciers, for example, the intermediate types may suddenly find 

 suitable habitats and become common. When crossing between two 

 species does occur, the result usually is not the swamping of the 

 original species but the enrichment of the variation of the parental 

 forms. Indeed, this appears to be very common in many genera of 

 perennial plants studied in the United States. 



Selection against Hybrids. It seems likely that, when highly 

 differentiated populations rejoin, selection operating against hybrid 

 individuals usually reinforces factors tending to prevent hybridiza- 

 tion. The exchange of genetic information between the isolates often 

 becomes negligible. It is entirely possible that, when more is known 

 about the processes of differentiation, it will be discovered that 

 hybridization between individuals of rejoining segregates is almost 

 universal, in other words, that mechanisms preventing exchange of 

 genetic material between differentiated forms usually arise only 

 through relatively unsuccessful hybridization after sympatry has 

 been reestablished. 



An interesting experimental demonstration of this mechanism was 

 obtained by Koopman, who synthesized artificial mixed populations 

 of Drosophila pseiidoobscura and D. persimilis and held them at 

 low temperatures (16°C) at which sexual isolation between the 

 two is at a low ebb (i.e., hybrids are formed more readily at low 

 than at high temperatures ) . Under the experimental conditions, the 

 hybrids were extremely unsuccessful, but Koopman intervened to 

 produce complete failure of hybridization by removing all hybrid 

 individuals before they could reproduce (hybrids were identified 

 by genetic markers ) . 



Over a period of several generations the proportion of hybrids 

 formed showed a marked decrease, indicating a reinforcement of 

 whatever factors were operating to prevent hybridization. Koopman 

 was able to show that the isolating mechanism was at least in 

 part sexual; i.e., males "preferred" to mate with females of their own 

 kind. 



In nature, D. pseudoobscura and D. persimilis, although occurring 

 in the same geographic areas, presumably do not hybridize for two 

 reasons. First of all, there is considerable ecological isolation, D. 

 persimilis usually occurring higher in the mountains and preferring 

 cooler, shadier spots than D. pseudoobscura. Sexual isolation must 

 also play a part, for except at low temperatures newly captured 



