Increase in Number of Species 195 



hybrid unit is formed. Before hybrid species can be formed, barriers 

 to interspecific crossing must be overcome, and factors favoring 

 the segregation of the hybrid progeny as a unit must be present. 



In plants, barriers to crossing are chiefly physiological or eco- 

 logical, and presumably passive chance leads to the production of 

 hybrid progeny. In animals, however, usually extensive behavior 

 patterns prevent interspecific mating. Miller (1955) suggested that 

 in birds these were circumvented if the population was so low that 

 individuals could not find mates of the same species. Spieth (1952) 

 and others noted that in sympatric species of Drosophila no mismat- 

 ings occurred in mixed cultures having goodly numbers of speci- 

 mens of each species. Hybridization was accomplished only by 

 confining males of one species with females of another. Sailer ( 1954 ) 

 found the same situation in attempting to cross two sympatric 

 species of stink bugs, Euschistus variohrius and E. servus. Even 

 when males and females of these opposing bug species were con- 

 fined together, matings were infrequent, oviposition was reduced, 

 and the per cent of egg fertilization was extremely low. A few 

 hybrids were finally produced, however, and they proved to be 

 vigorous and fertile. These hybrids possessed certain notable char- 

 acters: (1) they mated more readily with each other than with 

 either parent, and ( 2 ) hybrid to hybrid matings produced the larger 

 number of offspring. This latter point agrees with a few observed 

 interspecific hybrids in Drosophilu (Patterson and Stone, 1952). 



In the genus Amaranthus, Sauer (1957) found that various 

 hybrid combinations were selected by the environment and that a 

 certain hybrid type tended to predominate in each environment. 

 The same phenomenon has been observed in other plants (Steb- 

 bins, 1950; Clausen, 1951). Darlington (1958) pointed out that 

 certain genetic combinations resulting from hybrid crosses would 

 be selected on a developmental basis because of ontogenetic phys- 

 iological fitness. 



Considering these observations, the following circumstances 

 would favor the production of new species by hybridization: (1) 

 local breakdown of interspecific barriers to crossing, (2) the pro- 

 duction of hybrid combinations better fitted than the parents to 

 thrive in the area of hybridization, and (3) any factors tending to 

 bring about mating or more certain fertilization between hybrids 

 than between hybrids and parents. All of these would be more 

 likely to occur at the fringes of the range of two parental species, 

 and they suggest that peripheral hybrid populations may develop 

 into new phylogenetic lines distinct from the parental ones. 



