The Evolution of Communities 



241 



Fig. 103. Geographic distribution of the polyploid races of the Dicanthiinn 

 annulatitm grass complex. Stars indicate diploids (thought to represent tlie 

 ancesti'al home in and near India); circles indicate tetraploids; and "plus" signs 

 indicate hexaploids. (From Celarier, Mehra, and Wulf. ) 



one, and the hybrid in turn gave rise to a tetraploid species strongly 

 resembhng the diploid. The tetraploid, however, spread far beyond 

 the range of the diploid and eventually colonized practically 'the 

 entire Old World tropics from China to central Africa. On the 

 southern edge of its range in Africa the tetraploid apparently hy- 

 bridized with another species (possibly the related South African 

 D. aristatum). From this union a hexaploid species arose wbich 

 now has a range in South Africa to the south of the tetraploid 

 species. Thus through hybridization and polyploidy a species with 

 a small range has given rise to a series of polyploids that have 

 dispersed into and colonized a large area and undoubtedly a goodly 

 number of different ecological communities. 



SYMPATRIC ORIGIN OF NEW SPECIES 



As was mentioned in the preceding chapter, new species may arise 

 by various mechanisms other than geographic isolation: polyploidy 

 in certain plants and a few animals, host transfer mechanisms in 

 the case of host-specific species, displacement of seasonal thiiing 



