ary history. One tragic example is the ill- 

 fated Donner party of California pioneers, 

 who became trapped by snow with little 

 food during the winter of 1846-47. Half of 

 the males but only 5 percent of the females 

 in the age range from five to thirty-nine 

 died. Women can stay alive with less food 

 than men can because women are smaller. 

 In another tragic example, the first man to 

 collapse and die on Scott's disastrous trek 

 to the South Pole was the biggest, Edgar 

 Evans, starved by Scott's democratic divi- 

 sion of his limited food supply into equal 

 portions for each of his men regardless of 

 their differing weights. 



Selection against too much as well as 

 too Uttle biological investment results in 

 the fine-tuning of our design, depending 

 on the demands of our natural fives. Con- 

 sider, for instance, the fine-tuning of breast 

 number, which proves to be correlated 

 with the natural variation in litter size. 

 Most mammal species have a teat number 

 double the number of pups in their average 

 fitter, and equal to the number of pups in 

 their maximum natural litter. That is, 

 mammalian teat design has a safety factor 

 of 2 for normal operation. We fit that rule: 

 we have a safety factor of two breasts for 

 our usual fitter of one; we're prepared for 

 our occasional twin births, which account 

 for as much as 5 percent of births in some 

 human populations; but we make no pro- 

 vision for triplets and larger birth num- 

 bers, which were vanishingly rare before 

 modem fertility drugs. For all but those 

 rare mothers of triplets, four breasts would 

 merely add to our weight and operating 

 costs. The occasional appearance of super- 

 numerary teats in humans and other mam- 

 mals reveals our genetic potential for more 

 breasts, a potential that is evidentiy reined 

 in by natural selection. 



Innumerable other examples testify to 

 the ubiquity of such fine-tianing. Males of 

 those species that have sfightiy higher ex- 

 pected frequencies of copulation have 

 slightly larger testes. (That's why men 

 have bigger testes than gorifias but smaller 

 ones than chimpanzees). Birds and mam- 

 mals with higher metabolic rates have 

 slightly bigger hearts and kidneys than re- 

 lated species with lower metabolic rates. 

 Such fine-tuning affects every aspect of 

 our design, from the molecular level to the 

 level of the whole body. 



Physicists, and even many biologists, 

 scorn evolutionary biology as a descrip- 

 tive science, full of just-so stories and de- 

 void of predictive power. An oft-quoted 

 example of this prejudice is a notorious re- 

 mark by the physicist I. D. Rutherford, to 



the effect that you don't understand some- 

 thing until you can express it numerically. 

 While this remark is in many respects 

 wrong and ignorant, the critics still have a 

 valid point. Granted, it's harder to identify 

 and measure the numbers underlying bio- 

 logical safety factors than those underly- 

 ing safety factors of elevator cables. But 

 we evolutionary biologists deserve much 

 of the blame ourselves for not even trying. 

 The major challenge that I see for evo- 

 lutionary biologists in the coming decades 

 is how to convert a quafitative science into 

 a quantitative one. That requires estimat- 

 ing such factors as the costs of building. 



maintaining, and operating a kidney; the 

 variation in our kidneys' preprogrammed 

 waste-excreting capacity, depending on 

 damage and deterioration with age; and 

 our normal rates of production of wastes 

 to be excreted by the kidneys. Gathering 

 all that information is at least conceivable 

 in principle, even though it won't be easy. 

 But the prize for success is big. It's noth- 

 ing less than a quantitative understanding 

 of biological design. 



Jared Diamond is an evolutionary biolo- 

 gist and physiologist at the University of 

 California Medical School, Los Angeles. 



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