Isolating Mechanisms 523 



ern Alabama, and in the lower Mississippi Valley, where Pero- 

 myscus leucopus and P. gossypinus are found in the same region 

 and in the same habitats and where there is no evidence of their 

 mating except for two presumed hybrids found in Alabama. Dice 

 has assumed that these two species were once separated geo- 

 graphically and that while they were separated they diverged 

 psychologically to such an extent that they do not mate even 

 when they are found in the same region. 



Numerous other examples of isolation of species because of 

 mating preferences upon the part of the individuals of the two 

 species may be cited from fish, moths, spiders, snails, birds, and 

 Drosophila, but the result is always the same — a failure of mat- 

 ing between individuals that may otherwise be fertile together. 



Gamete Incompatibility Isolation. Two species may occupy 

 the same region and habitat, may be sexually active at the same 

 time, and may be separated by no mechanical or psychological 

 barrier, but yet may not produce a new generation of sexually 

 reproducing offspring. Even though they may mate, some other 

 barrier may be present that prevents the establishment of hybrid 

 swarms. One is the failure of fertilization after mating because 

 the sperm never reaches the egg. 



Gene-Cytoplasm Isolation. In the last class of isolation 

 mechanisms, the sperm never reached the egg. In some inter- 

 family crosses, however, as between echinoderms and annelids 

 or mollusks, the sperm penetrates the egg only to be later thrown 

 out or dissolved. The cause is an antagonism between the sperm 

 nucleus and the cytoplasm of the egg and acts as an isolating 

 mechanism by preventing the formation of species hybrids. 



Hybrid Lethality Isolation. In this class of isolating mecha- 

 nisms, the sperm enters the egg, the sperm and egg nuclei unite, 

 and the embryo proceeds to develop. The embryo, however, does 

 not usually develop very far, although in some organisms it may 

 develop into an adult which dies before it reaches sexual ma- 

 turity. Among animals this type of isolation has been reported 

 in fish, beetles, and moths. The cause apparently is a general 

 disharmony between the genes of the two parents, producing an 

 animal that is structurally so abnormal that it cannot function 

 properly and dies. 



In some species crosses in plants the same phenomena are ob- 

 served. The hybrid forms and dies at a very early stage. It may 



