172 I The Process of Evolution 



Outbreeding Systems 



Negative assortative mating is the mating of unlike individuals 

 with a frequency greater than that expected under random mating. 

 The differentiation of sexes in animals generally assures that self- 

 fertilization cannot occur. In addition, most animals have developed 

 systems of varying degrees of complexity which influence the degree 

 of outbreeding. These are briefly discussed in Chap. 5. There is 

 evidence that genetically controlled components of dispersal affect 

 outbreeding and gene flow in insects. Behavioral mechanisms in both 

 invertebrates and vertebrates often operate to reduce the frequency 

 of nearest-neighbor matin gs. In Homo sapiens such ethological 

 mechanisms reach their extreme. 



A diversity of mating types affects recombination in microor- 

 ganisms. Dispersal mechanisms in plants, as in animals, make near- 

 est-neighbor matings less frequent than would be expected if chance 

 alone determined the pairings. Flowering plants have floral pollina- 

 tion mechanisms which also function to determine the amount and 

 type of recombination. It is commonly assumed that there has been 

 a general evolutionary trend from open flowers, composed of numer- 

 ous parts, which are pollinated more or less indiscriminately, to 

 flowers with the few stamens and stigmas positioned in such a way 

 that pollen is precisely applied to and withdrawn from the body of 

 the pollinator. 



Other plants have physiological incompatibility systems that en- 

 sure outbreeding. Since reproduction in the higher plants requires 

 pollination by male gametophytes, as well as fertilization by male 

 gametes, the process can be interrupted at many steps. A common 

 system has a multiple allelic basis, the gene for incompatibility, S, 

 existing in many states. The various possible genotypes affect polli- 

 nation so that pollen tube growth is very slow in a style with the 

 same allele of S as is found in the male gametophyte. Other more 

 complex incompatibility systems have been studied in the flowering 

 plants. 



SUMIVIARY 



Recombination is a basic property of nucleic acids. Viruses and bac- 

 teria show simple, variable genetic systems producing recombination 

 with great wastage of recombinants. In flagellates and filamentous 

 algae, the life cycle may be predominantly haploid with a diploid 



