202 Increase in Number of Speciea 



Polyploidy offers unusual possibilities for the combination in one 

 line of characteristics possessed by two or more lines so different 

 that they would not produce fertile hybrids. Chiefly involved in 

 this role would be inter-group hybrids and inter-generic hybrids. 

 An example of the former is the grass Bromus carinatus complex 

 representing an allopolyploid between the sections Bromopsis and 

 Ceratochloa of Bromus; an example of the inter-generic type is 

 domestic wheat, an allopolyploid of the hybrid between Triticum 

 dicoccoides and Aegilops speltoides. Although naturally occurring 

 polyploids of proven inter-generic parentage are few, many diver- 

 gent types probably arose by this method. 



Polyploidy has been of tremendous significance in increasing the 

 number of plant species. The lowest possible estimate of polyploid 

 species is 25 per cent with the possibility of this figure reaching 

 35 or 40 per cent. The percentage differs widely in various plant 

 groups. In the mosses (Steere, 1954, 1958) and angiosperms (Steb- 

 bins, 1950) some families have little or no polyploidy, whereas 

 others have many examples; only in the ferns (Manton, 1950) are 

 the bulk of the families almost entirely polyploid. 



The many thousands of polyploid species do not each represent 

 a new phylogenetic line that arose by the process of polyploidy. 

 Thus in Clarkia (Fig. 34) eight of the ten polyploid species each 

 represent a separate polyploid origin, but the two species C. davyi 

 and C. teneUa both arose from a single polyploid line which divided 

 into the two sister species by isolation and non-polyploid genetic 

 divergence. In the horsetail genus Eqiiisetiim, the 11 fertile species 

 listed by Manton (1950) all have a gametic chromosome number 

 of 108, surely polyploid, but all 11 species have undoubtedly 

 evolved by ordinary non-polyploid processes from a common poly- 

 ploid ancestor having a gametic chromosome number of 108. 



Species and Lower Categories 



The natural processes leading to species fission are in operation 

 constantly but not synchronously. What we observe at the present 

 time is a cross-section through all the cvoKing phylogenetic lines 

 of the world. All stages in the species fission process are represented 

 in existing biotas. As a consequence difficulties arise in giving names 

 to these products. 



Among the phylogenetic lines diverging by the accumulation of 

 small genetic changes, some will be represented by mature species 

 which are demonstrably independent in an evolutionary sense, 



