322 The Process of Evolution 



Endoplasmic reticulum the system of tubes and vesicles in the cyto- 

 plasm of plant and animal cells, the membranes bounding which are 

 usually associated on the outside with ribosomes and are continuous 

 with the plasma membrane and the outer membrane of the nuclear 

 envelope. 



Endopolyploidy division of the chromosomes without division of the 

 nucleus, resulting in higher than zygotic chromosome number. 



Environmental variance that portion of the phenotypic variance caused 

 by differences in the environments to which the individuals in a popu- 

 lation have been exposed. 



Enzymes proteinaceous catalysts of cellular reactions. 



Epigenotype the series of interrelated developmental pathways through 

 which the adult form is realized. 



Epistasis interaction of nonallelic genes. 



Erythrocyte the type of hemoglobin-containing cell found in the blood 

 of vertebrates. 



Euchromatic describing those portions of the chromosomes that manifest 

 the usual prophase-telophase transformations and contain those genes 

 with major phenotypic effect. 



Eupolyploidy increase in chromosome number by whole genomes. 



Expressivity a measure of the uniformity of the phenotypic expression 

 of a gene in a particular environment. 



Feedback the influence of the result of a process upon the functioning 

 of the process. 



Fitness the survival value and reproductive capability of a given geno- 

 type relative to other genotypes in a population. 



Founder principle the principle which states that, when a new popula- 

 tion is established in isolation, its gene pool is not identical with that 

 of the parent population because of sampling error; these differences 

 are enhanced as different evolutionary pressures in the areas occu- 

 pied by the two populations will then also be operating in different 

 population genetic environments; and the result is increased diver- 

 gence. 



Gametocyte a cell which, through division, will form gametes. 



Gametophyte that phase of the life cycle in many plants in which the 

 gametes are produced, usually by mitosis. 



Gap a discontinuity in variation. 



Gastrula the stage in development following the blastula in which the 

 cells at the surface move to form an invagination (the archenteron), 

 which has an opening to the outside known as a blastopore. 



Gene the segment of a chromosome between two closest points of cross- 

 ing-over; a hereditary unit having more than one state and whose dif- 

 ferent states produce differences in the phenotype; a segment of genetic 

 material that bears the information specifying the structure of a single 

 protein or polypeptide chain. See cistron, recon. 



Gene How the movement of genetic information within and among 

 populations. 



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