118 Species and Species Change 



Reproductive Characteristics 



In both plants and animals, hybrids between sympatric species are 

 rarely observed in nature. Experimental evidence indicates that 

 almost invariably some physiological reaction, ecological circum- 

 stance, or mating behavior pattern prevents cross-mating betv^een 

 different species. Examples of physiological reactions include the 

 failure of pollen grains to grow properly on foreign pistils and the 

 dying of the sperm of certain Drosophila species when transferred 

 to the spermatheca or sperm storage gland of a foreign species. 

 Ecological circumstances preventing cross-breeding include dif- 

 ferent times of sexual maturation in both plants and animals, the 

 pollination of flowering species by different species of insects, and 

 the differing times of day during which species are sexually active. 

 If cross-mating does take place, varying degrees of intersterility 

 can be found between the species, ranging from complete inter- 

 sterility to moderate interfertility. In the latter cases, the hybrid 

 progeny are generally either sterile or have reduced viability. Thus 

 not one but many circumstances tend to reduce interbreeding be- 

 tween species. The proportionately rare occurrence in nature of 

 hybrids between sympatric species is evidence of the effectiveness 

 achieved by all these circumstances acting together. The result is 

 that each species is an interbreeding system continuing from genera- 

 tion to generation, sufficiently isolated reproductively from other 

 species that it maintains it own identity. Hence, although different 

 species may be able to exchange characters to a limited extent, each 

 species is independent in an evolutionary sense. 



Visible Identification Characteristics 



Almost without exception even closely related species differ in 

 some visible trait. Frequently these differences are of small mag- 

 nitude. A remarkable example was demonstrated by Price ( 1958 ) . 

 His studies showed that in eastern North America the fresh water 

 copepod commonly referred to as Cyclops vernalis comprises at 

 least seven species reproductively isolated from each other, three 

 differentiated by minute but constant characters, the other four 

 by only average differences. 



In some instances, the diagnosis of species requires techniques 

 involving special preparations and characters inaccessible to the 

 student of museum specimens. In the insects of the black-fly family 

 Simuhidae, for example, many apparently distinct genetic species 



