714 GENETICS AND EVOLUTION 



of man have appeared and become extinct in the past few hundred 

 tliousand years. In general, evolution tends to occur rapidly when a new 

 species first appears, and then gradually slows down as the group becomes 

 established and adapted to its particular environment. 



3. New species do not evolve from the most advanced and specialized 

 forms already living, but from relatively simple, unspecialized forms. 

 The mammals, for example, did not evolve from the large, specialized 

 dinosaurs, but from a group of rather small and unspecialized reptiles. 



4. Evolution is not always from the simple to the complex. There are 

 many examples of "regressive" evolution, in which a complex organism 

 has given rise to simpler ones. Most parasites have evolved from free- 

 living ancestors which were more complex than they; wingless birds, 

 such as the cassowary and emu, have descended from birds that could 

 fly; many wingless insects have evolved from winged ones; the legless 

 snakes came from reptiles with appendages; the whale, which has no 

 hind legs, evolved from a mammal that had the customary two pairs of 

 legs and so on. These are all reflections of the fact that mutations occur 

 at random, and not necessarily from the simple to the complex or from 

 the imperfect to the perfect. If there is some advantage to a species in 

 having a simpler structure, or in doing without some structure alto- 

 gether, any mutations which happen to occur for such conditions will 

 tend to accumulate by natural selection. 



5. Evolution occurs by populations, not by individuals; evolutionary 

 processes are brought about by the processes of mutation, natural selec- 

 tion and genetic drift. 



Questions 



1. What were Darwin's chief contributions to the theory of evolution? 



2. Discuss the essential points of Lamarckism. What has led to the rejection of this 

 theory? 



3. Discuss the advances in the science of geology that paved the way for the theory of 

 evolution. 



4. Describe in your own words what Darwin meant by natural selection. 



5. What changes in the theory of natural selection have been made necessary by dis- 

 coveries since Darwin's time? 



6. What contributions to the principle of evolution were made by Erasmus Darwin, 

 Alfred Russell Wallace, Thomas Huxley, Thomas Malthus and Hugo de Vries? 



7. Discuss the role of isolation in the origin of species. 



8. What is meant by "genetic drift"? Under what circumstances is it important in evolu- 

 tion? 



9. Discuss the theory of preadaptation. Describe two examples of preadaptation other 

 than the ones given in the text. 



10. Compare the Neo-Darwinian and the macromutation theories of the origin of species. 



11. Distinguish between the several types of mutations. What physical and chemical 

 agents are known to produce mutations in the laboratory? What agents may produce 

 spontaneous mutations in natural populations? 



12. What explanation may be given for the observation that most spontaneous and in- 

 duced mutations produce phenotypes which are less well adapted for survival than 

 the original form? 



13. Describe the steps by which simple inorganic substances may have undergone chem- 



