112 Natural Selection 



SELECTION IN APOMICTIC ORGANISMS 



In apomictic species, natural selection operates solely by its effect 

 on each individual. In these reproductive types there is no op- 

 portunity for genetic mixing between individuals. The members of 

 the species form clones or pure lines rather than an interbreeding 

 population. Selection in these species has certain distinctive fea- 

 tures. 



The genetic variability in one individual is thought to be ex- 

 tremely small. In asexual types it will be nil except for an occasional 

 viable diploid or polyploid mutant, but this will immediately give 

 rise to two homozygous types of offspring. In diploid unisexual 

 types undergoing meiosis, there is the theoretical possibility that 

 a considerable number of allelic heterozygotes would be formed, 

 but White (1954) believes that even in these forms homozygosity 

 predominates. If true, it is theoretically possible to delete a genetic 

 trait from the population by removing one certain individual. 



Because apomictic species lack a mechanism for accumulating 

 mutants from different parents in one individual offspring, maximum 

 rate of change in them should theoretically be much lower than 

 in bisexually reproducing types. If, however, an unusually well 

 adapted mutant arose it should be able to supplant older types 

 with great rapidity. 



SELECTION IN BISEXUAL POPULATIONS 



In bisexually reproducing populations natural selection acts by 

 removing individuals, but in one sense the population as a whole 

 is involved. The reason for this lies in the large store of variability 

 in an entire population, and the fact that in the mating process 

 an effective mixing of genetic variants takes place. It is highly 

 likely that ( 1 ) in most cases no offspring are exactly like either of 

 their parents and (2) the removal of one individual will not delete 

 any genetic trait from the genetic makeup of the population. 

 Selection, therefore, operates on the total gene pool of an inter- 

 breeding population. 



This interbreeding or Mendelian population, called a deme, may 

 be either small and local or large and regional, depending on the 

 continuity of the range and the ability of the organisms to disperse 

 or be dispersed (called their vagility). It may include the entire 

 population of the species or only one minute segment of the species. 

 The size of the deme is important in selection. 



