PROBLEM 5. Breeding New Types 



v-i 





Fig. 443 Two varieties of corn, developed at 

 the University of Illinois. Oiie tall and one 

 short plant were selected. Then, for several 

 corn generations, breeders planted seed froiJi 

 the tall offspring of the tall plants a?id the short 

 offspring of the short plants, (science service) 



fertilization is the usual method in soy- 

 beans, the breeder selects only one plant 

 as the parent for the next generation. 

 This is continued each season until he 

 discovers that the percentage of oil in 

 the seed no longer increases. He has then 

 reached the highest oil content that se- 

 lection can achiev'e with his soybean 

 plants. From this time on the breeder can 

 do nothing more than prevent cross- 

 pollination from soybean plants that may 

 have a lower oil content. He strives to 

 keep his plants at the same high level in 

 each generation. Try Exercise i. 



Improvement through selection stops 

 after a time. Until about twenty-five 

 years ago no one knew why selection pro- 

 duced improvement up to a certain point 



oj Animals mid Plants 495 



and then stopped. Now we can apply the 

 gene theory and explain the facts. Each 

 time the breeder selected plants with the 

 highest oil content in the beans, he was 

 choosing those plants that had the best 

 gene combination for oil production. 

 These "good" gene combinations tended 

 to be inherited when the plants were used 

 as parents. Over a number of generations 

 through selection the breeder can sepa- 

 rate plants with "good" combinations 

 from plants with "poor" combinations. 

 By consistently discarding the "poor" 

 ones he eventually has only the "good." 

 By using the plants with good combina- 

 tions as parents he can expect ofTspring 

 \\ ith the same good combinations, but he 

 cannot expect to get offspring that are 

 better than the "good" parents. 



Test your understanding of selection 

 as a method of improving organisms by 

 doing Exercises 2 and 3. 



Varieties and species. When the 

 breeder produces a soybean plant whose 

 seeds have a much higher percentage of 

 oil and the plant obtained is pure and 

 breeds true he has produced a new 

 variety or strain. In animals it is often 

 called a breed. The plant is different from 

 the original one but it is still a member of 

 the same species because the change is a 

 minor one. In this case there would be 

 no doubt in your mind about its belong- 

 ing to the same species as the soybean 

 plant whose seed contains little oil. 



But in many instances it would be im- 

 possible for you to determine whether 

 two organisms that differ are sufficiently 

 different to be considered separate species 

 or merely separate varieties within the 

 same species. It is difficult even for biolo- 

 gists to draw the line and they do not 



