Chapter 30 

 CYTOGENETICS AND EVOLUTION 



Species 



The concept of a species has undergone numerous changes from 

 the time it was beheved that each species was a specially created 

 entity until the present, when the frequent discoveries of numer- 

 ous hybrids and intergrading forms keep reminding us that spe- 

 cies change and that throughout the course of evolution various 

 species have appeared and disappeared. To define a species 

 according to our present factual information is by no means a 

 simple problem, and many different definitions have been offered 

 by various biologists from time to time. 



One of the more satisfactory definitions is A. E. Emerson's. 

 He defines a species as "a genetically distinctive, reproductively 

 isolated, natural population." He elaborates this by saying, 

 ''The genetic distinction may be morphological, physiological, or 

 behavioristic. The isolation, whatever the mechanism, effectively 

 prevents interbreeding with other populations. The population 

 concept emphasizes the interplay of biological factors between 

 the individuals." 



Unfortunately, however, most of the species that we know- 

 were established on the basis of one or several preserved speci- 

 mens in a museum and have not been studied or recognized with 

 any relationship to barriers, either biological or geographical. 



Although we may regard a species as a biological unit consist- 

 ing of a large number of plants or animals, we must understand 

 that the individuals comprising it are not homozygous or even 

 genetically identical. Since various members may be heterozy- 

 gous for several genes and since the self-fertilization or crossing 

 of various individuals may produce types that are phenotypically 

 different from the parents, species show variation which differs 

 in extent from species to species. Often certain genotypes may 

 become more prevalent in certain geographic localities of the 

 habitat of the species whereas other genotypes may become more 



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