ISOLATING MECHANISMS AND SPECIES FORMATION 



suggestive, but unfortunately there is much more evidence against it than 

 there is for it. On the positive side, a few interspecific crosses in moths 

 have been described in which death resulted from the inability of the 

 copulating pair to separate. Two snails of different species were observed 

 trying to copulate over a period of several hours, but they failed, presum- 

 ably because of mechanical difficulties. On the negative side, there is a 

 great array of evidence. In some cases, extreme differences in the genitalia 

 do not prevent successful copulation. Copulation between insects of strik- 

 ingly different morphology has been observed. In many genera, the female 

 genitalia may be identical throughout, while the males show differences 

 of taxonomic importance. Even extreme size differences may be no bar 

 to copulation: the Dachshund and the St. Bernard have been successfully 

 crossed. 



Character Displacement. An isolating mechanism which might be 

 discussed as restriction of random mating or as reduction of fertility is 

 character displacement, a concept recently developed by Brown and Wil- 

 son. They observed many cases in which closely related species are most 

 strongly differentiated where they are sympatric, yet convergent where 

 they are allopatric. Thus the ant Lasius flaviis has a wide holarctic distri- 

 bution, while L. nearcticiis is confined to northeastern U.S. In this com- 

 mon area, the two species are quite different both morphologically and 

 ecologically, yet elsewhere in its range, flaviis is rather similar to nearcti- 

 ciis. Their interpretation is that, when recently separated species become 

 sympatric, selection favors strong differentiation which will reduce the 

 probability of wastage of gametes by matings of low fertility. Also, eco- 

 logical differentiation reduces direct competition between the species, 

 thus permitting a greater total population in a given region. Character 

 displacement may influence any aspect of the biology of species, including 

 fertility. Brown and Wilson have cited many examples from groups as 

 diverse as birds and ants, fishes and crabs. 



Reduction of Fertility. The final category of isolating mechanisms 

 comprises those which act through a reduction of fertility. This is ordi- 

 narily subdivided into interspecific sterility, in which there is a failure to 

 produce an Fi, and hybrid sterility, in which a good Fi is produced, but 

 this hybrid is sterile, and so no Fo results. Yet the case is already over- 

 stated, for the phenomenon is not absolute: fertility may be reduced 

 without producing absolute sterility, and this is very commonly the case, 

 particularly among plants. The literature on interspecific sterility and 

 hybrid sterility is immense, going clear back to Aristotle. While it cannot 

 be summarized here, some of the more salient facts will be discussed 

 below. 



Interspecific sterility may be based upon the failure of the pollen or 

 sperm to reach the ovule or egg, or upon the production of an inviable 

 zygote. The first type is particularly well known in plants. It frequently 

 happens that the growth of pollen tubes is slowed down in interspecific 

 crosses, or that the pollen tube bursts, so that no fertilization is possible. 

 If the species crossed are only remotely related, the pollen tubes may not 

 grow at all. If they are more closely related, they are likely to grow more 



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