472 INTRODUCTION TO EVOLUTION 



the other is impossible. This so-called mechanical isolation is sometimes 

 encountered in insects with highly complex genital organs. A comparable 

 situation exists in those plants in which the flower structure is such that 

 cross-pollination cannot occur. (2) The breeding seasons of the two 

 species may not coincide. Flowers of one population of plants may open 

 and then disappear before the flowers of another population mature. One 

 population of insects in which adults live but a few days may be effectively 

 isolated from another population if the periods of emergence of adults in 

 the two do not coincide. 



In other cases interbreeding is possible but does not occur (sexual or 

 psychological isolation; ethological isolation). In many insects copulation 

 is preceded by rather elaborate courtship behavior. Closely related species 

 may differ in details of this ritual. Females readily accept only males 

 which observe the punctilios of courtship behavior characteristic of their 

 species. This behavior may be the expression of a form of preference in 

 the choice of mates. It clearly seems to be such in higher animals. Thus 

 related species of deer mice (Peromyscus) are found not to interbreed in 

 a state of nature, although some will do so readily enough when placed 

 in cages together, i.e., when possibility of choice is removed. 



That this tendency to selective or preferential mating may be a potent 

 force is suggested by experiments of Reed and Reed (1950). We noted 

 (p. 459) an earlier experiment by these investigators in which it was 

 found that when two genetic types of fruit flies were placed in competition 

 an equilibrium was reached, the more deleterious genetic type not being 

 completely eliminated from the population. Quite otherwise were the 

 results of another experiment in which fruit flies having the mutant gene 

 for "white eye" were placed in competition with normal wild-type (red- 

 eyed) flies. In twenty-five generations the white-eye gene disappeared 

 from the population completely. The investigators determined that 

 white-eyed flies were not less viable than were wild-type individuals. Tests 

 indicated, however, that the mating behaviors of white-eyed and red-eyed 

 flies were strikingly different. Both red-eyed and white-eyed females 

 "preferred" to mate with red-eyed males. The strength of this preferential 

 mating was determined, and expressed mathematically, calculations show- 

 ing that it was sufficient to account for the disappearance of the white-eye 

 gene in the number of generations within which that elimination was ob- 

 served to occur. (See also "Hybridization: Disadvantageous," pp. 474- 

 477.) 



Reproductive isolation may exist even in cases in which matings be- 

 tween populations take place. ( 1 ) Fertilization may not occur as a result 



