Genetic Systems II [ 207 



polyploidy are carried to what appear to be extremes, the situation 

 is not, as usually described, "dead end." The pollen of obligate 

 apomicts may be functional, and pollination of an apomict may occa- 

 sionally result in the segregation that leads to an escape from 

 asexuality. 



SUMMARY 



Populations of plants and animals often exhibit cytogenetic mech- 

 anisms controlling the amount and nature of genetic recombination. 

 These mechanisms, along with others previously mentioned, make 

 up the genetic system of the population that determines how many 

 new gene combinations are produced in a unit of time. They range 

 from inversions and translocations, which produce relatively small 

 groups of linked genes, through polyploidy with its diverse and vari- 

 able eflfects, to apomixis, in which recombination is eliminated. Such 

 mechanisms are often considered disadvantageous in the very long- 

 range view. However, they are extremely common in both plants 

 and animals and must result in a selective advantage. The bizarre 

 and complicated genetic systems of some organisms discussed are 

 poorly understood and have not been satisfactorily integrated into 

 evolutionary theory. 



REFERENCES 



Darlington, C. D. 1958. The Evolution of Genetic Systems. 2d ed. Basic 

 Books, Inc., New York. A remarkable attempt to unify cytology and 

 genetics in evolutionary terms. 



Sager, R., and F. J. Ryan. 1961. Cell Heredity. Wiley, New York. This is 

 a recent source for material on the genetic systems of microorganisms. 



Stebbins, G. L. 1960. The comparative evolution of genetic systems. In 

 Sol Tax [ed.], Evolution After Darwin, Univ. Chicago Press, Chicago, 

 vol. I. The Evolution of Life, pp. 197-226. A thorough recent account 

 of the problem, together with speculations concerning answers. 



White, M. J. D. 1954. Animal Cytology and Evolution. 2d ed. Cambridge 

 Univ. Press, New York. The standard reference for genetic systems in 

 animals, though now somewhat out-of-date. Other references to animals 

 will be found in Evolution After Darwin, vol. 1, cited above. 



