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codes remain largely undeciphered in our 

 genes. The biological arena in which nat- 

 ural selection operates is life itself, with its 

 myriads of competing individuals and spe- 

 cies. Yes, if all other things were equal, we 

 would be better off with larger kidneys, 

 more breasts, thicker bones, and more en- 

 zymes. But one cannot ignore the price of 

 those benefits. All biological structures 

 incur direct costs of biosynthetic energy 

 and indirect costs of space occupied. En- 

 ergy in the form of ATP is required not 

 only to make a molecule or organ in the 

 first place but also to maintain and operate 

 it. Big organs incur further, indirect oper- 

 ating expenses because of the weight that 

 has to be lugged around, as anyone who 

 has been overweight or pregnant knows. 



But the food energy available to an ani- 

 mal is finite. Space is also finite, as you 

 may have appreciated when you saw all 

 those organs packed closely together in- 

 side the body cavity of the frog that you 

 dissected in introductory biology. Since 

 available energy and space are limited, 

 any resource devoted to one organ or en- 

 zyme comes at the expense of another. 

 Thus, an economically designed animal 

 will tend to outcompete not only an under- 

 equipped animal but also an overequipped 

 one, profligate in one organ and necessar- 

 ily shorted on some other. 



The potential disadvantages of biologi- 

 cal underdesign are obvious. Small bones 

 break when overstressed, and small kid- 

 neys on which you dump too many toxins 

 can't protect you against poison. As for 

 the potential disadvantages of biological 

 overdesign, they're reflected in the evolu- 

 tionary loss or reduction of organs that be- 

 come unnecessary because of an evolu- 

 tionary change of life style. For example, 

 why is it that so many birds on remote, 

 predator-free islands have small wings or 

 no wings, when flight seems so obviously 

 advantageous? The hundreds of flightless 

 bird species that evolved on Hawaii, Mau- 

 ritius, and other islands testify to the ad- 

 vantages of getting rid of expensive and 

 heavy flight muscles, which account for 

 up to one-quarter of a bird's mass, when 

 they are no longer needed to escape preda- 

 tors. Other analogous examples include 

 the loss or reduction of eyes in cave- 

 dwelling animals, and the loss of nutrient- 

 synthesizing enzymes in bacteria grown in 

 mediums providing those nutrients with- 

 out cost. 



For humans, the disadvantages of 

 overdesign become clear whenever food 

 availability is limited, as it has been for 

 most people throughout human evolution- 



80 Natural History 6/94 



