196 Increase in Number of Species 



Spontaneous Genetic Isolation 



From time to time the possibility has been discussed that indi- 

 viduals might acquire a non-polyploid mutation which was fertile 

 when crossed with itself but sterile when crossed with the parent 

 stock and that such mutations would result in the production of a 

 new species. The theoretical probal^ilities militating against such 

 a happening are great. In a completely bisexual cycle, if only one 

 mutant individual occurred, it would be unable to produce off- 

 spring because it would have no mate except from the parent stock. 

 The occurrence of two such identical mutations in the same place 

 at the same time, and the mating of these two particular individuals, 

 seems highly improbable. 



Theoretically species fission of this type can occur only when the 

 mutant gametes can be increased in number between their forma- 

 tion and the next mating, hence might be possible in many unicel- 

 lular organisms having asexual reproduction interspersed with sexual 

 unions or in self-compatible, self-pollinated plants. 



Laven (1957, 1959) suggested spontaneous genetic isolation to 

 explain the occurrence of cryptic or obscure species of European 

 mosquitoes. In the house mosquito Ciilex pipiens he found (1957) 

 that populations from northern Germany produce no offspring 

 when crossed with populations from extreme western Europe, that 

 the latter were intersterile with Italian and Tunisian populations, 

 and that many other combinations of populations from different 

 areas exhibited varying degrees of intersterility (Fig. 86). No 

 morphological or other distinguishing attributes have yet been 

 found between these intersterile forms, nor does any mating prefer- 

 ence exist between them. The net result is the delineation of at least 

 five rigidly defined but contiguous areas each having a population 

 intersterile with its neighbor. In a genetic sense tlie population of 

 each area is a true species with a sharp lioundary between them. 



These boundaries are maintained by sterilit)' factors carried in 

 the cytoplasm (Laven, 1957). Laven (1959) suggested that these 

 arose within the parent population and, by increasing without 

 natural selection, formed the pr(\sent system of cryptic species. 



Several points militate against this view. In northern Germany 

 especially, local populations exhil)it xarious degrees of interfertility, 

 indicating that the intersterility factors arose as a succession of 

 small mutants or as the accumulation of dosages of an original 

 mutant having only slight effect. Normally such mutations would 

 be swamped out or would graduall)' spread through the whole 



