PROBLEM 



4 What Theories Have Been Offered to 

 Explain the Origin of the Different 

 Kinds of Animals and Plants? 



De Vries's contribution. In the preceding 

 problem you learned many reasons for 

 believing that animals and plants were 

 not always like those existing now; that 

 the first animals and plants that appeared 

 on the earth were simple; and that they 

 have given rise throughout the ages to 

 more complex forms. Our problem now 

 is to discover how it might be possible 

 for an organism to give rise to another, 

 resulting in the origin of new species. 



De Vries made an important contri- 

 bution toward an explanation of the ori- 

 gin of new species. Early in this century 

 he noticed that inheritable changes oc- 

 curred in the evening primrose plants 

 that he was studying. His experiments 

 showed that the changes were sudden. 

 He called them mutations. De Vries 

 noticed that some of these plants con- 

 tinued to mutate. If such mutations were 

 to continue through long periods of geo- 

 logical time many new characters would 

 appear. Thus, said De Vries, entirely 

 new organisms that could be classed as 

 new species could arise by the addition 

 of many small mutations. De Vries's ex- 

 planation of how evolution takes place 

 is called the mutation theory. 



Effects of gene changes. Gene changes 

 (or other chromosomal changes) in the 

 germ plasm may lead to characters 



which weaken the organism; then the 

 organism dies and fails to produce off- 

 spring having the new character. Most 

 gene changes that we know about are of 

 this kind. These would never produce 

 ne\\' species. Some gene changes, how- 

 ever, seem to have little or no effect on 

 the organism's ability' to survive; a new 

 sunflower plant may have red blossoms 

 instead of yellow like its parents. But 

 some changes make the new organism 

 more fit to meet the conditions of hfe 

 and therefore to leave offspring with this 

 desirable change. For example, a wheat 

 plant that, because of a mutation, ac- 

 quires the ability to resist a disease w ill 

 live and reproduce when its neighbors 

 that cannot resist the disease may die 

 and fail to reproduce. In time resistant 

 wheat plants will establish themselves. 

 The nonresistant ones may not all die 

 out, however. They may never be at- 

 tacked by the disease or they may have 

 some other highly desirable character 

 which compensates for their weakness. 

 Thus both varieties may continue to 

 live. 



Changes may have complex effects; 

 it is not easy to say whether or not a 

 change will in the end help to produce 

 a new type of organism. See Exercises i 

 and 2. 



