318 j The Process of Evolution 



Apogamety agamospermy in which cells other than the egg form the 

 new embryonic sporophyte. 



Apomixis all forms of reproduction in which meiosis and syngamy are 

 partially or completely circumvented. 



Apospory agamospermy in which somatic cells of the sporophyte pro- 

 duce a diploid gametophyte through mitotic divisions. 



Arrhenotoky parthenogenesis resulting in the formation of haploid male 

 offspring, the females being diploid. 



Articular a bone found in the jaw of many vertebrates which presumably 

 has been modified into the malleus of the mammalian ear. 



Artificial selection the choosing by man, as far as possible, of the geno- 

 types contributing to the gene pool of succeeding generations. 



Asexual reproduction apomixis, q.v.; often used to refer to vegetative 

 propagation. 



Autocatalysis promotion of a reaction by its end products. 



Autopolyploid a polyploid formed by increase in the number of identical 

 genomes in the same organism or by the fusion of diploid or polyploid 

 gametes from organisms with essentially identical genomes. 



Autosome a usually euchromatic chromosome that is not a sex chromo- 

 some. 



Autotetraploid an autopolyploid with four similar genomes. 



Autotriploid an autopolyploid with three similar genomes. 



B chromosome a supernumerary heterochromatic chromosome found in 

 varying quantities in some plants (not necessarily in all cells). 



Backcross the mating of an offspring to one of its parents or to a parental 

 type. 



Bacteriophage virus attacking bacterial cells, often resulting in their 

 lysis. 



Balanced polymorphism polymorphism maintained in a population usu- 

 ally by the heterozygotes, at the locus under consideration, having a 

 higher adaptive value than either homozygote. 



Barrier a physical or psychological condition that hinders or prevents the 

 movement of individuals ( and thus of genetic information ) . 



Basal granules cytoplasmic organelles of characteristic fine structure to 

 which cilia and flagella are attached. 



Bimodal population a population with measurements of a given charac- 

 ter clustered around two values. 



Bivalent the synapsed pair of chromosomes in the meiotic prophase. 



Blastopore an invagination in the surface of an embryo in the gastrula 

 stage. 



Blastula an early stage in embryonic development in which the cells are 

 often arranged in a hollow sphere. 



Bradytelic evolution evolution at a much slower rate than horotelic evo- 

 lution. 



Breeding size the number of individuals in a population that are actu- 

 ally involved in reproduction in a given generation. 



Buffering protection of a system from change by outside forces. 



