Genetic Systems II 171 



drift. Small population size also leads to inbreeding and a deviation 

 from randomness of mating when the species is considered as a 

 whole. 



Inbreeding Systems 



Inbreeding may be referred to as positive genetic assortative mating 

 for it increases the chance of mating by organisms with like geno- 

 types. Mating systems leading to inbreeding result in the break-up 

 of a population into smaller groups that only rarely exchange genetic 

 information. The heterozygosity of the population is reduced as 

 fixation occurs in the subgroups. Genetic variance is increased in 

 the population as a whole unless there is strong selection for par- 

 ticular homozygotes. Morphological and physiological mechanisms 

 leading to inbreeding are common in plants. Their degree of re- 

 striction ranges from facultative self-pollination to obligate self- 

 fertilizing types with cleistogamous flowers (see Chap. 9). 



Detailed studies by Stebbins, Grant, and others have shown that 

 the degree of restriction of recombination in plants is closely corre- 

 lated with their growth form and habitat. As is discussed in Chap. 9, 

 populations often combine genetic systems that have opposite effects 

 on recombination. Herbaceous plants, which have short generations 

 and thus more recombination, tend to have low chromosome num- 

 bers. Perennial plants with longer generation time have higher 

 chromosome numbers. Plants that occur in ecological communities 

 usually thought of as "closed," i.e., those in which most offspring do 

 not survive to maturity, tend to have genetic systems that promote 

 genetic recombination. Oaks are an example. On the other hand, 

 pioneer organisms, wherever they may occur, are members of "open" 

 communities. In order for zygotes to survive, they must have the 

 proper genotvpes; there is no time for the organism to experiment 

 with recombinants. It is not surprising, therefore, to find that plants 

 of desert regions, grasslands, and cleared areas in the tropics 

 generally have genetic systems that result in reduced recombina- 

 tion. 



When inbreeding is imposed on populations that are usually 

 outbreeding, a loss of fitness referred to as inbreeding depression 

 occurs. The relationship between fitness, heterozygosity, and out- 

 breeding is not well understood as yet. Many groups of plants have 

 successfully employed inbreeding as a mating system for long pe- 

 riods of time with only occasional outbreeding. It cannot be assumed 

 that all organisms necessarily maintain developmental and genetic 

 homeostasis through outbreeding and heterozygosity. 



