RESTRICTIVE PRACTICES 



(J? 



I/) 



o 



a: 

 u 



o 



< 



CO 



>- 



U 

 < 

 o. 

 < 



U 



z 



a: 



CQ 



>- 



EXTERNAL and ENVIRONMENTAL 



1. GEOGRAPHICAL— separation in range of occurrence 



2. ECOLOGICAL— separation in habitat occupied 



3. SEASONAL— separation in time of occurrence or 



breeding 



INTERNAL and GENETIC 



1. PREVENTION OF MATING OF ZYGOTES by 



(i) Discrimination in Mating 

 (ii) Mechanical Inability 

 (iii) Failure of an Essential Intermediary (as where 



insects fail to carry pollen between two species 



of plants) 

 (iv) Prevention of all Crossing in Hermaphrodites (by 



devices securing self-mating) 



2. INCOMPATIBILITY of 



(i) Gametes 



(ii) Male Gamete and Female Soma (owing to genetical 

 unlikeness) 



FERTILIZATION 



UNBALANCE OF 



1. F, ZYGOTE— leading to Inviability and Genotypic 



sterility 



2. Fi GAMETES— resulting from Genotypic effect of F^ 



zygote or, in plants, from Segregational sterility 



3. F2 ZYGOTES— resulting, in animals, from Segregational 



sterility 



Fig. 77. — Isolating mechanisms preventing the effective exchange of genetic 

 materials between populations. They fall into two groups: Bars to Crossing acting 

 before fertilization and capable (in so far as they are genetic) of being produced by 

 the direct action of selection; and Hybrid Incapacity occurring after fertilization, 

 and incapable of being produced by the direct action of selection. The two groups 

 are not mutually exclusive: they tend to encourage one another's occurrence. 



307 



