THE ADAPTATION OF ANIMALS TO THE ENVIRONMENT 783 



There are dogs and deer, adapted for terrestrial life in which running 

 rapidly is important for survival; squirrels and primates, adapted for 

 life in the trees; bats, equipped for flying; beavers, otters and seals that 

 maintain an amphibious existence; the completely aquatic whales, por- 

 poises and sea cows; and the burrowing animals, moles, gophers and 

 shrews (Fig. 38.2). The number and shape of the teeth, the length and 

 number of leg bones, the number and sites of attachment of muscles, 

 the thickness and color of the fur, and the nails, claws or hoofs at the 

 tips of the toes are some of the structures which are involved in adapta- 

 tion. In Australia, where there were no placental mammals until very 

 recently when they were introduced by man, the marsupials underwent 

 a comparable adaptive radiation to fill the different habitats there. With 

 a little study the many unusual animals of Australia can be recognized 

 as the ecologic equivalents of the more familiar animals native to the 

 United States. 



352. Convergent Evolution 



The animals living in the same type of habitat tend to develop 

 structures which make them superficially alike, even though they may 

 be but distantly related. This evolution of similar structures by animals 

 as they become adapted to similar environments is known as convergent 

 evolution, or adaptive convergence. The dolphins and porpoises (which 

 are mammals), the extinct ichthyosaurs (which were reptiles) and both 

 bony and cartilaginous fishes have evolved streamlined shapes, dorsal 

 fins, tail fins and flipper-like fore and hind limbs which make them look 

 very much alike (Fig. 38.3). Seals and penguins have streamlined shapes 

 and flipper-like limbs but lack the dorsal and tail fins of the other aquatic 

 animals. Moles and gophers, in adapting to a burrowing life, have 

 evolved similar fore and hind leg structures adapted for digging, but the 

 mole is an insectivore and the gopher is a rodent. 



Figure 38 3 Convergent evolution. All of these aquatic \ertebrates have a marked 

 superficial similarity despite their distant relationship, because of their adaptations to 

 similar environments. 



