258 



CHAPTER 29 



that the genetic changes which lead to specia- 

 tion are not accomphshed merely by the ac- 

 cumulation of more mutants of the kinds 

 which distinguish races. What is required 

 for speciation are mutants with special ef- 

 fects, effects which contribute to reproductive 

 isolation. Populations usually must be 

 physically separated while reproductive bar- 

 riers are being built up, otherwise hybridiza- 

 tion would break down these barriers. There 

 is also experimental evidence, in support of 

 the hypothesis described earlier, that natural 

 selection, acting both directly and indirectly, 

 will itself further the accumulation of genetic 

 factors promoting reproductive isolation be- 

 tween races. 



We have discussed, so far, how one species 

 can give rise to two or more species, via races 

 which serve as incipient species. Note that 

 in defining a species, it was necessary to say 

 that it had an isolated gene pool, that is, a 



PARENT 

 SPECIES 



W 



n 



DIPLOID 

 HYBRID 



gene pool closed to individuals of some 

 other alternative condition (species). There 

 is one possible type of species formation 

 which would be unrecognized by this cri- 

 terion, since the alternative state would no 

 longer exist. It is conceivable, for example, 

 that a species composed of a single popu- 

 lation would gradually undergo numerous 

 changes in its gene pool during the course of 

 many generations. At the end of this time 

 should we call the new population the same 

 species, or should we give it a new species 

 name? It is likely that, in some cases, had 

 some of the members of the original popu- 

 lation been miraculously preserved, the old 

 form would be found reproductively isolated 

 from the new. In this event we would be 

 deahng with the formation of a new species 

 whose origin was dependent upon the ex- 

 tinction of an old one. The occurrence of 

 this type of speciation will become a valid 

 subject of study, once man learns how to 

 preserve sample genotypes indefinitely. 



Not only may new cross-fertilizing species 

 originate from a single species or its races, 

 but many have arisen as the consequence of 

 hybridization between two or more different 

 species. What we are dealing with here are 

 the consequences of interspecific liybricliza- 

 tion. We already know that if interspecific 

 hybrids are formed, they pose no threat to 

 the isolation of the gene pools of their paren- 

 tal species. The question we are asking is: 



Aneuploid 



Melotic 



Products 



AMPHIPLOID 



Euploid(n) 



Meiotic 



Products 



FIGURE 29-2. Interspecific hybridi- 

 zation leading to new species forma- 

 tion via amplnploidy. 



