CHAPTER 12 



BREEDING SYSTEMS 



Breeding Systems Iiibreediiio and Outbreeduio Devices Incompatibility 

 Hctcrostyly Mating Discrimination The Breakdown of Control 

 Stratification The Inbreeding Hybrid Apomixis 



The gametes which pair and fuse in sexual union can differ from 

 one another in two ways; in form and in origin. In regard to form, 

 there is a complete range, from the eqiiaUty of some algae, fungi 

 and protista, to the vast inequality between male and female germ 

 cells reached in birds and reptiles. This extreme of differentiation 

 represents a division of labour, for it enables the organism to combine 

 the mobility of the sperm, which makes fertilization possible, with 

 the food storage of the egg, which gives the embryo a start in life. 

 Like other differentiation, this difference between the male and 

 female gametes is not to be traced to any corresponding difference 

 in tlie genes which the cells carry. Gametic differentiation can be 

 as complete when the gametes are genetically alike as when they 

 are genetically unlike; when they are produced by the same 

 individual as when they are produced by different ones. 



Of greater genetical significance is the origin of the gametes. 

 Whether they are equal or unequal, the two which fuse may come 

 from the same, or from different, diploid individuals. And if, as 

 in most animals, from different individuals, these may have different 

 degrees of relationship with one another. Where, as in some plants 

 like Johannsen's beans, they regularly come from the same indi- 

 vidual, generation after generation, the product oi fusion wiU be 

 homozygous, or nearly so. Where, on the other hand, they come 

 from different individuals the product will be in some measure 

 heterozygous or hybrid. 



Now, in particular populations or mating groups of plants and 

 animals there will be an average degree of hybridity. This will 

 foUow from the average relationships of the parents. And this in 

 turn will depend on a variety of conditions of which the most 

 obvious and most important is the relative frequency of self- and 



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