248 I The Process of Evolution 



clusters of clones that owe their similarity to interclone selection. 

 One of the enduring controversies among evolutionary theorists 

 concerns the possibility of sympatric speciation by sexual organisms. 

 Certainly, as stated above, the vast majority of evidence indicates 

 allopatric speciation to be the rule, and many cases that have been 

 presented as evidence for sympatric speciation are easily explained 

 on other grounds (e.g., the cichlid-species swarms). However, one 

 argument against sympatric speciation is that gene flow will swamp 

 out any differences produced by a disruptive selection pressure. 

 There has recently been some provocative work by Thoday and his 

 coworkers and by Streams and Pimentel which indicates that this is 

 not necessarily so. These workers have shown in laboratory experi- 

 ments with Drosophila that disruptive selection can produce diver- 

 gence in the absence of isolation. For instance, Thoday and Gibson 

 subjected a wild-type population to disruptive selection for chaeta 

 number, with both high and low selected individuals being placed 

 in a common vial for mating. At the end of 12 generations the 

 original population had split into two populations, which produced 

 few hybrids. Therefore, sympatric speciation may not be theo- 

 retically impossible, but its significance in nature is yet to be deter- 

 mined. As Slobodkin has aptly stated: 



In one sense, the distinction between theoretician, laboratory worker, 

 and field worker is that the theoretician deals with all conceivable worlds 

 while the laboratory worker deals with all possible worlds and the field 

 worker is confined to the real world. The laboratoiy ecologist must ask 

 the theoretician if his possible world is an interesting one and must ask 

 the field worker if it is at all related to the real one. 



In the sympatric-speciation controversy the ball seems to have been 

 passed to the field worker. 



SUMMARY 



Available evidence indicates that differentiation in isolation is the 

 primary source of organic diversity. Populations physically sepa- 

 rated from each other (so that gene flow is minimized or absent) 

 have different evolutionary "experiences" and thus differentiate 

 genetically. If this process proceeds beyond a certain point, the 

 populations will not reunite if contact is once again established. The 

 investigation of this genetic "point of no reunion" and the develop- 

 ment of generalizations concerning it are among the most difficult 



