336 INTRODUCTION TO EVOLUTION 



Indeed, natural selection always operates on whole animals, never on sepa- 

 rate parts of animals. Hence in a given environment the successful animal 

 will be the one that combines the greatest number of qualities tending to 

 adapt it to the environment in question. Accordingly, the shuffling of the 

 genes is a means of producing a continuous supply of new combinations to 

 be "tried out" by natural selection. Deleterious combinations are weeded 

 out; beneficial combinations are favored. In this way progress, in terms of 

 adaptation to environment, is made. 



But continual reshuffling has its drawbacks as well as its utility. Once a 

 beneficial combination of genes is formed will it not be broken up again 

 in the very next generation by continuation of the shuffling process which 

 created it? It is a matter of common observation that children do not inherit 

 all the characteristics of one parent. A particularly gifted parent seldom if 

 ever can endow a son or daughter with all the attributes which combined 

 to produce his own unusual talent. To a considerable extent this recombin- 

 ing of characteristics does serve to break up favorable combinations. 



There are, however, hereditary mechanisms tending to decrease the fre- 

 quency of such dissolution of combinations. We have mentioned the fact 

 that genes are contained in visible structures called chromosomes. One chro- 

 mosome contains many genes, and the genes in one chromosome tend to 

 stay together in inheritance. The tendency is, however, counteracted by 

 processes making possible exchange of genes between chromosomes. These 

 processes (crossing over and translocation, pp. 395-398) contribute to the 

 reassorting of genes but they act with relative slowness. More, they are some- 

 times prevented or still further slowed by other genetic factors (such as the 

 presence of inversions — see pp. 397-398). The subject is a complicated 

 one, involving for its understanding considerable knowledge of the mecha- 

 nisms of heredity; these receive further attention in Chapter 17. The point 

 we wish to make here is that mechanisms preventing, or at least hindering, 

 the breaking up of beneficial combinations of genes do exist. 



In summary, in the reassorting and recombining of genes lies a means 

 whereby natural selection is provided with raw materials in the form of 

 varied combinations of hereditary characteristics. The diversity is increased 

 whenever populations having somewhat difTerent genes interbreed. We 

 must next inquire how the differing genes arise in the first place. 



NEW GENES FROM OLD 



As noted earlier (Chaps. 2 and 5), genes occasionally 

 undergo a change called mutation. In its commonest form this is a chemi- 

 cal change in the gene; as a result the changed or mutant gene has an efTect 



