418 INTRODUCTION TO EVOLUTION 



nonadaptive traits may be accounted for by the side effects of pleiotropic 

 genes (pp. 350-351) or by the random fixation of genes through genetic 

 drift (pp. 349,439). 



POLYPLOIDY IN EVOLUTION 



In the preceding chapter we discussed the essentials of 

 the genetical phenomenon of polyploidy, that type of chromosomal aberra- 

 tion in which the number of chromosomes is increased by multiples of the 

 haploid number (pp. 399-400). What role has polyploidy played in evo- 

 lution? 



Polyploids may arise in various ways from diploid ancestors. Perhaps the 

 easiest way to visualize is that by means of the production of diploid germ 

 cells by these ancestors. Ordinarily germ cells contain one haploid set of 

 chromosomes. But sometimes owing to abnormality in the process of germ 

 cell formation the full number of chromosomes characteristic of the par- 

 ent's cells finds its way into one germ cell. Thus, a parent whose diploid 

 number is eighteen will ordinarily produce germ cells containing nine 

 chromosomes (i.e., nine chromosomes comprise one haploid set in this 

 species). But occasional germ cells may be produced which contain all 

 eighteen chromosomes. If a diploid ovule of this kind is fertilized by a 

 diploid pollen grain (also containing eighteen chromosomes), the result is 

 a fertilized ovum containing thirty-six chromosomes. These thirty-six 

 chromosomes will comprise four haploid sets (two from the male parent 

 and two from the female) and the resultant individual will be a tetraploid. 

 A tetraploid is likely to differ from its diploid ancestors in a number of 

 ways, presenting quite an altered appearance. Many tetraploids are en- 

 tirely fertile among themselves, or self-fertile (we recall that production of 

 both pollen and ovules by a single plant is not unusual) . But most important 

 of all, there is frequently a high degree of sterility between the polyploid 

 and its diploid progenitors. Thus reproductive isolation arises at one 

 stroke, and without the aid of spatial isolation. This point is of interest 

 since usually, as we have emphasized elsewhere (pp. 473-474), spatial 

 isolation is important in the first stages of species formation. 



Polyploidy has formed a common means of speciation in plants. Multi- 

 tudes of wild species are polyploids, as are many of our cultivated plants: 

 cotton, wheat, oats, tobacco, potato, banana, coffee, sugar cane, many of 

 our cultivated flowers, and so on. Indeed, plant breeders are today con- 

 stantly employing artificial means of inducing polyploidy to create new 

 varieties. There is historical interest in the fact that some of the "muta- 



