196 I The Process of Evolution 



rapid evolution: subunits partially isolated in space which occasion- 

 ally exchange genetic material. It makes possible further exploita- 

 tion of the advantages of hybridization at the diploid level. 



APOMIXIS 



The ultimate in the restriction of recombination is the elimination of 

 sexual reproduction altogether. In extreme cases, recombination is 

 completely eliminated. This appears to be relatively rare, and there 

 are many types of asexual reproduction with varying amounts of 

 recombination. The term apomixis describes all types of asexual 

 reproduction, that is, those types of reproduction that tend to re- 

 place or act as substitutes for sexual reproduction. The classification 

 of the types of apomixis is exceedingly complex, since a variety of 

 situations intermediate between sexuality and obligate apomixis 

 may occur. In most plants, either the gametophytic or the spo- 

 rophytic generation or both may be involved. Terminology relating 

 to apomixis also has become unfortunately complex; it will be sim- 

 plified as much as possible in the following account. 



Apomixis may be facultative or obligate. Some organisms repro- 

 duce sexually at times and asexually at other times, usually under 

 different environmental conditions. However, in animals, a regular 

 cyclic change from sexual to asexual reproduction is not strictly 

 facultative apomixis, since the asexual portion of the cycle is obli- 

 gate. 



The simplest sort of apomixis is vegetative reproduction; it may 

 function as the only mode of reproduction. For example, some 

 aquatic plants are known to reproduce only asexually in northern 

 Europe (e.g., Elodea, where only pistillate plants are found) and 

 sexually in other parts of their range. The self-sterile triploid day 

 lily (Hemerocallis fulva) reproduces asexually in America. The 

 effect of such vegetative propagation, whether by rooting and 

 further growth of broken-off branches or by the production of spe- 

 cialized propagules such as gemmae or bulbils (which are modified 

 branches or buds), is to create a population of genetically identical 

 organisms. Such a population is called a clone. Clones may be 

 formed by animals that reproduce by fission ( Protozoa ) or budding 

 ( Coelenterata, Annelida, etc.). From the standpoint of genetics, the 

 population is one genetic individual. Since they usually eventually 

 separate from one another, members of a clone are individuals 

 physically and play the role of individuals ecologically. 



Among both plants and animals, vegetative reproduction may 

 occur in individuals that also reproduce sexually and thus have a 



