250 Origin of Biomes and Succession 



pendently and bringing about an unequal increase in the number of 

 species in different phylogenetic lines within a community. The 

 mechanism based on rate of genetic change would bring about a 

 disproportionate increase in the number of species in lines which 

 changed rapidly. The mechanism based on breadth of ecological 

 tolerance would bring about a disproportionate increase in the 

 number of species having narrower ecological tolerances. 



EVOLUTION FROM COMMUNITY TO BIOME 



As the process of community splitting and fusing continues, more 

 and more species are mixed or at least brought into contact. This 

 action increases the possibilities for additional sympatric species 

 fission through host-transfer mechanisms, hybridization, and subse- 

 quent allopolyploidy. It also adds more complexities to the predator- 

 prey and competitive relationships. It seems highly unlikely that this 

 mixing of species would result in a simple addition of the two sets 

 of species because in certain of the highly competitive groups some 

 species would surely become extinct. Simpson (1947, 1950) ex- 

 pressed the opinion that in the mammals the extinction following 

 faunal interchange is considerable. It seems likely that if this high 

 rate of extinction does occur in the mammals, it may be associated 

 with relatively large absolute size, combative habits, and the be- 

 havior pattern called territoriality because evidence from many in- 

 sect groups agrees better with the concept of a remarkably small 

 degree of extinction following a mixing of species (Ross, 1957). 



Such extinction as does occur following faunal mixing is usually 

 attributed to competition for food or living space or to changes in 

 the available physical environment which exceed changes in eco- 

 logical tolerances made by the species. Breeding habits are another 

 factor which may lead to extinction. If two species come together 

 which are genetically incompatible but not sexually isolated from 

 each other, one of them might be obliterated from progressive areas 

 of range overlap. This is happening in the case of two European 

 hares, Lepus timidus and L. europeus, whose ranges abut. L. timidus 

 is a montane and northern species and L. europeus is a more wide- 

 spread lowland form. When the latter was introduced into the 

 northern range of L. timidus in Sweden, L. europeus became estab- 

 lished there and eliminated the local population of L. timidus. The 

 mechanism for this elimination was discovered experimentally by 

 Notini (1941, 1948), who found that the two species cross-mate 

 freely but the hybrids are sterile. Because the mating season of L. 



