326 I The Process of Evolution 



Mutagenic agent an agent leading to an increase in mutation rate above 

 the spontaneous level at a locus. 



Muton a mutational site within a gene. 



Natural selection nonrandom (differential) reproduction of genotypes 

 without the conscious intervention of man. 



Neurula the stage in a vertebrate embryo following the gastrula, char- 

 acterized by rapid differentiation. 



Niche the status (way of life) of an organism in a community; its 

 position in the food chain, relationships with the physical substrate, 

 etc.; related to the concept of adaptive zone. 



Nondisjunction failure of the chromosomes in a bivalent or multivalent 

 to separate at the first meiotic anaphase. 



Nuclear envelope the outer boundary of the nucleus, composed of two 

 perforate unit membranes, the outer of which is continuous with the 

 endoplasmic reticulum. 



Nucleic acids complex acids composed of nucleotides. 



Nucleotide a phosphate ester of the 2V-glycoside of a nitrogenous base; 

 chemical building block of a nucleic acid. 



Oligogenes those genes with major obvious phenotypic effect; switch 

 genes. 



Ontogeny the development of an individual. 



Operator a postulated genetic element that acts as a receiver of spe- 

 cific cytoplasmic signals in the form of repressor-substance molecules 

 and that controls an operon. 



Operon a unit of linked cistrons, the expression of which is controlled 

 by an operator; a genetic unit of transcription of the DNA code. 



Organelle a structure of characteristic morphology and function within 

 the cytoplast; the unicel analogue of an organ in a multicellular or- 

 ganism. 



Organizer a portion of an embryo that determines the developmental 

 fate of the cell masses with which it comes into contact. 



Oriented meiotic divisions meiotic divisions with the spindle oriented 

 so that a particular group of chromosomes always enter a polar body 

 rather than the egg. 



Overdominance the result of the heterozygote being more extreme than 

 either homozygote. 



Oxidation a reaction in which oxygen is acquired or hydrogen is lost by 

 a compound or in which the valence of the metallic element is raised. 



Pairing segments the euchromatic regions of chromosomes that synapse 

 with a homologue in the first meiotic prophase. 



Panmixis random mating. 



Paracentric applied to inversions that do not include the centromere. 



Parallelism convergence among closely related forms. 



Parthenogenesis the development of an individual from an unfertilized 

 gamete. 



Pectate a compound of pectic substances (acid polysaccharide amor- 

 phous carbohydrates) . 



