320 I The Process of Evolution 



Cistron an operational unit, equivalent to or smaller than a genetic 



region, controlling a specific protein. 

 Cleavage division one of the early divisions of the zygote that leads to 



the formation of an embryo in animals. 

 Cleistogamous producing inconspicuous flowers that never open and are 



thus self-pollinated. 

 Climax community the relatively stable community following a suc- 



cessional series; usually thought of as characteristic of a particular 



climatic zone. 

 Cline a gradient of variation in the measurement of a character of a 



population or in a complex of characters of a population, the gradient 



often varying in steepness along its length or being stepped. 

 Clone a population derived by asexual reproduction of a single indi- 

 vidual. 

 Coacervates aggregates of varying degrees of complexity, resulting from 



the interaction of two or more colloids. 

 Coenzymes substances which combine with specific proteins to form 



complexes that are catalytically active. 

 Colloid a dispersion of one substance within another, having properties 



of both a solution and a suspension. 

 Community the group of organisms found in a particular place. 

 Competition use of the same limited resources by two or more organisms. 

 Conjugation union of cells or organisms during which all or part of the 



genetic information of one individual is transferred to another. 

 Conspecific considered by taxonomists to belong to the same species. 

 Convergence superficial resemblance resulting from occupancy of sim- 

 ilar adaptive zones. 

 Cretaceous the last period of the Mesozoic; beginning about 130 mil- 

 lion years ago. 

 Crossing-over the exchange of usually corresponding segments between 



chromatids of homologous chromosomes; results in chiasmata and gene 



recombination. 

 Culture a body of nongenetic information transmitted from generation 



to generation. 

 Cybernetic involving a control or governing mechanism operated by 



feedback from the process. 

 Cytoplasmic inheritance inheritance of traits whose determinants are 



not located on the chromosomes. 

 Cytosine the pyrimidine (2-hydroxy-4-aminopyrimidine) involved in the 



structure of both DNA and RNA. 

 Decay of variability the reduction of heterozygosity because of genetic 



drift leading to loss and fixation of alleles at various loci. 

 Deletion the loss of a segment of a chromosome. 

 Denatured protein a structurally modified protein showing decrease in 



solubility and change in biological activity. 

 Deoxyribonucleic acid ( DNA ) giant molecules which in most organisms 



are the nuclear repository for the genetic information and which are 



