420 INTRODUCTION TO EVOLUTION 



Sinnott, Dunn, and Dobzhansky, 1958). The radish (Raphanus) and the 

 cabbage (Bnissica) both have a diploid number of eighteen, a haploid 

 number of nine. The hybrid is a tetraploid, having received two haploid 

 sets of radish chromosomes and two haploid sets of cabbage chromosomes 

 (total: thirty-six) (Fig. 18.6). Since the hybrid is infertile with both parent 

 species it forms a new species; this has been named Raphanobrassica by 

 combining the names of its parents. The hybrid is intermediate between 

 its parents in many characteristics; the intermediate structure of its seed 

 pod is clearly evident in Fig. 18.6. Unfortunately Raphanobrassica seems to 

 have no commercial future, since it combines the root of a cabbage with 

 luxuriant leaves resembling those of a radish! 



If space permitted, other examples of polyploidy as a means of evolution 

 in plants might be described. Many polyploid species have arisen in a state 

 of nature, and frequently it is possible for botanists to decide which diploid 

 species were their ancestors. The matter has been "clinched" in a few 

 cases by actually "resynthesizing"' the polyploid, starting with the diploid 

 species suspected of being its parents. 



Although polyploidy is most important in the evolution of plants (for dis- 

 cussions see Dobzhansky, 1951, and Stebbins, 1950), it has been of almost 

 no importance in animal evolution. It is found in few animals and among 

 that few only in some types which have forsaken reproduction involving 

 two sexes (bisexual) for reproduction in which a single individual produces 

 both ova and sperm (hermaphroditism) or for reproduction by means of 

 unfertilized eggs (parthenogenesis). Apparently the sex-determining mech- 

 anism of animals cannot function properly when polyploidy occurs. 



In sum, we find that plants possess in polyploidy one means, and per- 

 haps the only means, by which a new species can arise in one step, achiev- 

 ing reproductive isolation in the absence of spatial isolation. Mayr (1949) 

 has termed this phenomenon "instantaneous speciation." 



GENETIC ASSIMILATION AND 

 THE BALDWI N EFFECT 



In Chapter 15 we stressed the importance of natural 

 selection operating upon randomly occurring mutations, and in subsequent 

 discussions we shall emphasize this action again (pp. 450-480). While this 

 is the most generally recognized means of evolutionary change, many in- 

 vestigators suspect that it is not the only means. The perfection with which 

 organisms are adapted to their environmental niches has led these peo- 

 ple to despair of explaining adaptation upon a basis of natural selection 



