328 I The Process of Evolution 



Polysaccharide large carbohydrate molecules resulting from the com- 

 bination of many smaller carbohydrate and possibly other molecules. 



Polytene chromosome a giant chromosome consisting of many chromo- 

 some strands closely associated along their lengths. 



Population a set of items in which one is interested; the individuals in 

 a given area at a given time; in many biological discussions, a syno- 

 nym of mendelian population, although this is often unstated. 



Population biology the study of the pattern in which organisms are re- 

 lated in space and time and including such disciplines as ecology, tax- 

 onomy, behavior, population genetics, and others that deal primarily 

 with the interactions of entire organisms. 



Population structure the sum of all the factors that govern the pattern 

 in which gametes from various individuals unite with each other. 



Porphyrins a class of compounds derived from pyrrole nuclei which, in 

 metal chelate complexes, form pigments important in many biological 

 processes. 



Primitive an organism or character judged to be less changed from a 

 presumed common ancestral state than another with which it is com- 

 pared. 



Probability density function a function that describes how probability 

 is distributed among the possible events, that is, the expected relative 

 frequencies of the events. 



Protein a class of compounds of high molecular weight formed by the 

 combination of amino acids with or without other molecules. 



Proteinoid a synthetic polypeptide. 



Protist a unicellular organism. 



Pseudocopulation a mode of pollination in some orchids in which struc- 

 tures of the flower resemble a female insect and male insects attempt- 

 ing copulation transfer pollen from one flower to another. 



Pseudogamy parthenogenetic development of an ovum after stimulation 

 (but not fertilization) by a male gamete or gametophyte. 



Purine an organic base made up of two condensed heterocyclic rings, 

 a pyrimidine ring and an imidazole ring. 



Pyrimidine an organic base consisting of a six-membered ring system 

 with nitrogen atoms at the 1,3 positions. 



Quadrate a bone found in the jaw of many vertebrates which presum- 

 ably has been modified into the incus of the mammalian ear. 



Quantum evolution rapid evolutionary change resulting in what a tax- 

 onomist would regard as a new higher taxon. 



Random sample a subset of a population selected so that all items in 

 the population are equally likely to be included in the sample. 



Recapitulation the idea that in the course of development an individual 

 passes through stages similar in form to adults of its presumptive an- 

 cestors; often stated as "ontogeny recapitulates phylogeny." 



Recessiveness the converse of dominance; a completely recessive allele 

 is expressed only in the homozygous state. 



