170 I The Process of Evolution 



property of DNA, this is perhaps an inappropriate designation. 

 Sexuality and diploidy probably evolved relatively early in time. 

 Reduction of recombination takes many forms. In unicels there may 

 be absence of sexual reproduction. In the more complex multicels, 

 reduction of recombination may occur as a result of reduction or 

 elimination of crossing-over, assumption of specialized mating sys- 

 tems, inbreeding, self-fertilization, or loss of sexuality. As Stebbins 

 has pointed out, such modifications are found most often in pioneer 

 forms whose populations experience pronounced fluctuations in 

 size. Under conditions of environmental change, such forms can ex- 

 ploit newly opened habitats through rapid duplication of closely 

 similar genotypes. Often there is reduction in body size and in 

 developmental complexity, as well as increased reproduction rate. 

 Cytogenetic mechanisms affecting recombination are discussed in 

 Chap. 9; systems of mating will be considered here. 



Since the amount of recombination is affected by selection, the 

 genetic system of nearly any organism is always in a state of flux. 

 The simultaneous existence of variable mechanisms producing in- 

 crease and decrease in recombination provides a system buffered 

 against short-term environmental change but able to respond to 

 long-term change. Most of the mating systems are bivalent in this 

 sense or are combined with other mechanisms to produce this bi- 

 valence. 



Mating Systems and Recombination 



The basic type of mating against which the others may be compared 

 is random mating. If like individuals are more apt to mate than 

 would be expected by chance, the system is said to be positively 

 assortative. If unlike individuals mate more frequently than expected 

 by chance, the system is negatively assortative. The nature of repro- 

 ductive mechanisms in plants and animals makes it unlikely that 

 truly random mating ever occurs. It is difficult to specify precisely 

 the extent of deviations from randomness. Mating systems do not, 

 of course, fall into discrete classes, and any population in nature 

 may show several of the arbitrarily delimited types. Furthermore, 

 mating-system type is affected by such things as selective advantage 

 of particular characters and population size. 



As pointed out in Chap. 6, random mating results in constant 

 gene frequency with no change in variability. With selection, gene 

 frequencies change, but the variance of the population and genetic 

 correlation between relatives are little affected. As the population 

 size reaches lower limits, the effects of sampling error lead to genetic 



