The Source of Variability 83 



EFFECTIVENESS 



A combination of crossing-over, chromosome segregation, and 

 parental diflFerentiation thus provides mechanisms which can theo- 

 retically produce every possible combination of alleles in a breeding 

 population. So great, however, is the total number of alleles in an 

 individual that tremendous populations and large numbers of 

 generations would be necessary to bring about this complete mixing. 



The mixing process is theoretically random and complete in 

 species having bisexual reproduction. In other species the mixing 

 process ranges from being more restricted to nonexistant. In or- 

 ganisms which may be facultatively self-fertilized, as many plants 

 may be, genetic mixing between individuals is restricted to those 

 individuals of the species which are cross-fertilized. No genetic 

 mixing occurs between individuals in obligate self-fertilized or in 

 apomyctic species. In self-fertilized species, free recombination of 

 the genes of a single individual is theoretically possible, as it is also 

 in types of parthenogenesis involving meiosis. In all other types of 

 reproduction, such as simple fission involving mitosis or various 

 types of vegetative reproduction, no genetic mixing occurs. The 

 name apomixis is usually applied to all types of reproduction in 

 which normal fertilization does not occur and means, literally, 

 "without mixing." 



Free and complete mixing of the genes in a population demands 

 that all progeny be fertile and that they cross back and forth with 

 each other. This appears to be the case in a species population in 

 any one locality. Hybrids arising from crosses between individuals 

 of distant populations may have greatly reduced fertility, and 

 hybrids between distinct species very frequently have low fertility 

 or none. Hybrids between distinct genera or higher categories are 

 seldom fertile. In any group of organisms there is a point of 

 taxonomic divergence beyond which no effective hybridization 

 occurs. Beyond this point no gene mixing is possible. 



The greater the genetic difference of hybridizing parents, the 

 more the characters are inherited as blocks from each parent, and 

 the fewer are the block combinations which produce fertile hybrids 

 (Darlington, 1958). Thus hybridization at the point of near-in- 

 compatibility between parents tends to produce progeny having 

 mosaics of the parental characters rather than intermediate condi- 

 tions of these characters. This situation may be exemplified by the 

 leaf hopper Erythroneiira alicia (Fig. 32), thought to be a hybrid 

 between £. metopia and E. trivittata or their immediate antecedents. 



