400 



INTRODUCTION TO EVOLUTION 



from species to species, however. The somatic (body) cells of man each 

 contain forty-six chromosomes, for example. These are comprised of 

 twenty-three pairs. Following meiosis each sperm and ovum contain 

 twenty-three single chromosomes. Thus in man a haploid set of chromo- 



Triploid 



Tetraploid 



FIG. 17.14. Polyploidy. Formation of a triploid fertilized ovum when a haploid 

 sperm cell fertilizes a diploid ovum (upper diogroms). Formation of a tetraploid 

 fertilized ovum when a diploid sperm cell fertilizes a diploid ovum (lower dia- 

 grams). 



somes numbers twenty-three. In the fruit fly, Drosophila, a haploid set 

 numbers four. 



Polyploidy seems to have been important in the evolution of plants, as 

 discussed below (pp. 418-420). 



Gene Mutations 



Evidence accumulates that genes consist of molecules of deoxyribose 

 nucleic acid (DNA). These are very large molecules having complex 

 structure. When the molecular structure is altered the result is a gene 

 mutation. On the whole, gene mutations produce more fundamental 

 changes than do chromosomal aberrations, and hence are of more im- 

 portance to evolution. 



Genes exert their control of living processes by controlling the nature or 

 production of enzymes. As we have seen, enzymes catalyze all living 



