544 - Heredity and Evolution 



though the more obvious external pheno- 

 typic effects of such mutations are partially 

 disadvantageous to survival. Such internal 

 effects seldom involve any visible changes of 

 structure, but presumably they involve 

 changes, however slight, in the chemical 

 composition or ultramicroscopic structure 

 of the tissues. They are usually referred to 

 rather vaguely as changes in the "viability" 

 or "fertility" of the stock. But despite the 

 fact that thev are invisible and very difficult 

 to study, these internal variations are most 

 important. Natural selection must — and de- 

 monstrably does — deal with many such in- 

 tangible characteristics. 



External adaptations include all the visible 

 characteristics of the individual that fit it 

 to survive and reproduce in its own particu- 

 lar environment. The shaping of all external 

 structures with reference to food getting, 

 respiration, self-defense, protection, repro- 

 duction, and so forth, all fall into the cate- 

 gory of external adaptations. But even more 

 important, perhaps, is the external behavior 

 of the organism: its responses to particular 

 environmental situations. Natural selection 

 has preserved individuals and races whose 

 responses are useful — to themselves, of course 

 — and has eliminated those whose behavior 

 was less favorable to survival. Accordingly, 

 it is found that most unconditioned responses 

 — tropisms, instincts, etc. — are strikingly 

 adapted to the conditions of the natural 

 habitat of the particular organism. 



The environment of any organism is an 

 extremely complex set of conditions. It in- 

 cludes not only the many physical and chem- 

 ical conditions of inanimate nature, but also 

 the multitude of living things with which 

 each organism comes into various kinds of 

 relations — predatory, parasitic, symbiotic, 

 competitive, cooperative, and so forth. In 

 fact, the study of organisms in relation to 

 the factors of their environment, which is 

 called ecology, represents an important 

 branch of biology (see Chap. 30). So complex 

 are these ecological interrelations and so deli- 

 cate is the balance between the processes of 



multiplication and destruction in any given 

 species, that the very slightest factor may tip 

 the scales, determining whether the indi- 

 vidual, the race, or the species shall perish or 

 survive. Without most careful study, there- 

 fore, it is very risk)' to judge that a particular 

 characteristic has no survival value for the 

 organism. On the other hand, it is not neces- 

 sary to assume that every visible characteris- 

 tic — such as the humps of the camel, the 

 bumps of the wart hog, or any other peculi- 

 arity of form, color, or habit — must neces- 

 sarily have some adaptative significance. In 

 many cases such features are merely inci- 

 dental effects of genes that have less obvi- 

 ous but more important effects upon the in- 

 ternal functions of the organism. 



STABILITY OF THE SPECIES 



Since most mutations are not favorable, 

 natural selection acts to preserve a high de- 

 gree of uniformity in each species by elimi- 

 nating a majority of these heritable changes 

 as fast as thev appear. Wild species display 

 less variability than domesticated animals 

 and plants. In the case of domesticated spe- 

 cies, man has interrupted the natural struggle 

 for existence and has artificially preserved a 

 number of less viable variants, which, under 

 natural conditions, could not have survived 

 in competition with their wild-tvpe relatives. 

 But even wild species are genetically hetero- 

 geneous for many genes with relatively slight 

 phenotypic effects, because natural selection 

 tolerates a number of slight mutations, even 

 though it almost always eliminates the larger 

 ones. 



Provided the environment does not change, 

 any species that has occupied a particular 

 habitat for a great many generations becomes 

 about as well adapted to conditions as its 

 inherent genetic nature will permit. In other 

 words, after a long time, the possibility of 

 new favorable mutations becomes rather 

 small. Under these conditions, therefore, 

 most species are fairly stable with reference 

 to the ai'eragc of their characteristics, al- 



