LECTURES IN BIOLOGICAL SCIENCES 



The proviso necessary for such an occurrence is that useless traits 

 must be by-products of the same genetic constitution which 

 yields also traits of overriding usefulness. Examples are not 

 hard to find either in human or in animal or plant evolution. 

 In man, the erect body posture has become established in human 

 evolution because it freed the hands from walking duties, and 

 thus permitted their employment for delicate manual operations. 

 The adaptive value for the human species of the possession of 

 well-developed hands is sufficiently obvious. But the erect body 

 posture in man entails disharmonies in several bodily functions. 

 The difficulty of childbirth is perhaps the most evident of these 

 disharmonies, and it is clearly disadvantageous from the stand- 

 point of natural selection. 



Why, then, has the erect posture become a fixed trait in the 

 human species? In a sense, this is just another manifestation of 

 the opportunism of natural selection. We often speak of natural 

 selection as favoring this and discriminating against that trait. 

 In reality, the selection perpetuates some and fails to perpetuate 

 other genetic constitutions, and not traits. It is the organism as 

 a whole that survives and reproduces, or remains childless and 

 dies. Therefore, possession of a very useful quality may compen- 

 sate for some concomitant weaknesses." Our species is a bio- 

 logical success because of its intellectual capacity. Man is bio- 

 logically specialized to control his environment by the force of 

 his powerful brain, not by that of his relatively weak body. The 

 self-awareness is, biologically, an adjunct of this powerful brain. 

 Man's biological success became a reality despite the tragic dis- 

 cords within him. 



It may seem odd that the genetic equipments of so many 

 living organisms, both lower and higher ones, show obvious, and 

 even glaring, imperfections. This is not what romantics gener- 

 ally expect to find in nature's products. But this is what might 

 be anticipated among the products of a natural creative process. 

 Organic evolution is such a process. It lacks foresight and the 

 ability to construct things according to a predetermined plan. 

 I like to illustrate the principle that the organization of living 

 beings is not free from shortcomings by the example of the man- 



8 T. Dobzhansky, American Naturalist 90, 1956. 



106 



