330 The Process of Evolution 



Speciation the splitting process of evolution, responsible for the exist- 

 ence of different kinds of organisms that are classified as species by 

 taxonomists. 



Species a group of organisms judged by taxonomists (by diverse cri- 

 teria) to be worthy of formal recognition as a distinct kind. 



Spindle the ellipsoidal mass of protein fibers visible during cell division 

 and thought to play a role in chromosome movement and division of 

 the cytoplasm. 



Spontaneous generation see abiogenesis. 



Stabilizing selection selection in which genotypes closer to the mean for 

 a character have an advantage over those at the extremes. 



Stationary frequency distribution the representation of a probability 

 density function showing the way the probability in a given situation 

 is distributed over the possible events. 



Structural gene the DNA code unit whose primary product is messen- 

 ger RNA and which thus controls synthesis of a particular protein or 

 polypeptide. 



Structural hybridity heterozygosity for a chromosomal rearrangement. 



Subspecies a geographic subdivision of a species deemed worthy of 

 formal recognition by a taxonomist. 



Succession the sequence of transient communities occurring in an area 

 before the climax (q.v.) community. 



Supergene a group of genes inherited as a unit. 



Supernumerary chromosomes chromosomes present, often in varying 

 numbers, in addition to the characteristic relatively invariable comple- 

 ment. 



Switch gene see oligogene. 



Sympatric occurring in the same geographic area. 



Sympatric speciation speciation without geographic isolation. 



Synapsis the association of homologous chromosomes in the meiotic 

 prophase. 



Syngamy union of gametes to form a zygote. 



Systematic pressure one of the nonrandom evolutionary pressures: se- 

 lection, mutation, or migration. 



Tachytelic evolution evolution at a much faster rate than horotelic 

 evolution. 



Terminalization the movement of chiasmata to the ends of the chromo- 

 somes during the later stages of the first meiotic prophase. 



Territoriality the defense of an area against organisms of the same or 

 similar kind. 



Tetraploid containing four genomes in reproductive cells other than 

 the gametes. 



Tetrasomic describing an organism with one chromosome in the com- 

 plement represented four times. 



Thallus a plant body not sharply differentiated into root and shoot. 



Thelytoky a form of apomixis in animals in which diploid females are 

 produced from unfertilized eggs. 



