Ahiogenesis the origin of life from nonliving systems. 



Acentric fragment a fragment of a chromosome lacking a centromere. 



Acrocentric chromosome a chromosome with a terminal or nearly ter- 

 minal centromere. 



Adaptive radiation evolutionary diversification, often over a relatively 

 short period of time, of a group of organisms, presumably following 

 their entry into a new adaptive zone; also used with reference to struc- 

 tures. 



Adaptive value the survival value and reproductive capability of a given 

 genotype relative to other genotypes in the population. 



Adaptive zone the "way of life" of a taxonomic group of organisms, in a 

 broad sense; may be subdivided into adaptive subzones. 



Adenine one of the two purines (6-aminopurine) involved in the struc- 

 ture of DxNA and RNA. 



Adenosine diphosphate (ADP) a compound made up of adenine, the 

 five-carbon sugar ribose, and two phosphate groups, which is involved 

 in the mobilization of energy in cellular metabolism. 



Adenosine triphosphate ( ATP ) ADP with an additional phosphate group 

 attached by a high-energy bond; decomposition of ATP to ADP makes 

 energy available for other reactions. 



Adventitious embryony production of an embryonic sporophyte by 

 mitotic divisions from tissues of another sporophyte without an inter- 

 vening gametophytic generation. 



Agamic without gametes; used with reference to complexes of organisms 

 in which all individuals reproduce asexually. 



Agamospermy formation of seeds without fertilization, with male gam- 

 etes, if present, serving only to stimulate division of the zygote. 



Aggregate any grouping of more than one object. 



Allele one of the several alternative states of a functional gene unit. 



Allometry different growth rates in different parts of the same organism. 



Allopatric with nonoverlapping geographic ranges. 



Allopatric speciation differentiation of populations in geographic isola- 

 tion to the point where taxonomists recognize them as separate species. 



Allopolyploid a polyploid formed by increase of chromosome number in 

 an individual with more than one type of genome. 



Allotetraploid a polyploid formed by doubling of chromosome number 

 in a diploid hybrid between two organisms with different genomes or 

 by the fusion of diploid gametes of such organisms. 



Amino acids the chemical building blocks of proteins; on hydrolysis, 

 proteins yield amino acids. 



Amphidiploid a tetraploid organism that is diploid for two genomes, 

 usually from different species. 



Aneuploidy increase or decrease of chromosome number by values less 



than whole genomes. | 317 



