728 GENETICS AND EVOLUTION 



Many species of animals have organs or parts of organs wfricfi are 

 useless and often small or lacking some essential part; in related organ- 

 isms, the organ is full-sized, complete and functional. There are more 

 than 100 such vestigial organs in the human body, including the ap- 

 pendix, the coccyx (fused tail vertebrae), the wisdom teeth, the nictitat- 

 ing membrane of the eye, body hair, and the muscles that move the ears 

 (Fig. .H5.-1). Such organs are the remnants of ones which were functional 

 in the ancestral forms, but when some change in the environment ren- 

 dered the organ no longer necessary for survival it gradually became 

 reduced to a vestige. This appears at first glance to be an application of 

 Lamarck's idea of the role of "use and disuse" of an organ in evolution, 

 but the underlying mechanism is quite different. Mutations for the 

 decrease in the size and functional importance of an organ are occurring 

 constantly; as long as the organ is necessary for survival, such mutations 

 are lethal and eliminate their possessors. But if the organ is no longer 

 needed for survival, such mutations will not be lethal and they may 

 accumulate and lead to the reduction of the organ. 



312. Evidence from Comparative Physiology and Biochemistry 



The study of the physiologic and biochemical traits of organisms 

 generally requires complex apparatus and is more difficult than the 

 direct observation of morphologic characters. Yet, as such studies have 

 been made using a wide variety of animal types, it has become clear that 

 there are functional similarities and differences which parallel closely 

 the morphologic ones. Indeed, if one were to establish taxonomic rela- 

 tionships based on physiologic and biochemical characters instead of on 

 the usual structural ones, the end result would be much the same. 



The fundamental similarity of the chemical constituents and pat- 

 terns of enzymes present in cells of different animals was presented in 

 Chapter 4. There are, however, certain chemical constituents, certain 

 enzymes and certain hormones that are found in some animals and not 

 in others. The distribution of these biochemical characters strongly 

 parallels the evolutionary relationships inferred from other evidence. 



The blood serum of each species of animal contains certain specific 

 proteins. The degree of similarity of these serum proteins can be deter- 

 mined by antigen-antibody reactions. To perform the test, an experi- 

 mental animal, usually a rabbit, is injected with a small amount of the 

 serum, as, for example, a sample of human serum. The proteins of 

 the injected serum are foreign to the rabbit's blood and hence act as 

 antigens, stimulating the production of antibodies which are specific for 

 human serum antigens. These antibodies are then obtained by with- 

 drawing blood from the rabbit and allowing it to clot; the antibodies 

 are in the serum. \\^hen a dilute sample of this serum is mixed with a 

 drop of human serum, the antibody for human serum reacts with the 

 human serum antigen and produces a visible precipitation. The strength 

 of the reaction can be measured by making successive dilutions of the 

 human serum, mixing each dilution with a fresh sample of the antibody 

 solution (the rabbit serum), and observing at what point the precipita- 



