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ANIMALS AND THEIR ENVIRONMENT 



long as that environment is present, but are at a great evolutionary dis- 

 advantage when the environment changes. In the course of time, organ- 

 isms have had to become readapted many times as their environment 

 changed or as they migrated to a new environment. As a result, many 

 animals today have structures or physiologic mechanisms that are useless, 

 or even somewhat deleterious, but which were useiul lor survival in 

 earlier times when the organism was adapted for a rather different en- 

 vironment. 



351. Adaptive Radiation 



The competition for food and living space tends to make each group 

 of organisms spread out and occupy as many different habitats as they 

 can reach and which will support them. The evolution from a single 

 ancestral group of a variety of forms which occupy different habitats is 

 called adaptive radiation. In this way organisms tap new sources of 

 food and escape from some of their enemies. The placental mammals 

 provide a classic example of adaptive radiation, for from a primitive, 

 insect-eating, five-toed, short-legged creature that walked with the soles 

 of its feet flat on the ground have evolved all of the present-day types. 



Figure 38.2. Adaptive radiation. All the various mammals have evolved from a 

 common ancestral insectivore. As they have evolved they have become adapted to a wide 

 variety of environments. The insectivores also underwent evolution, resulting in a number 

 of specialized forms such as the mole shown in the center. (Villee: Biology.) 



