THE CONCEPT Of EVOLUTION 703 



itself may change as time passes, a trait which has adaptive value at one 

 time may be useless or even detrimental at another. 



It has only recently been appreciated that chance, as well as natural 

 selection, may play a role in evolution. Chance may play a role in de- 

 termining whether a new mutation will be passed from the individual 

 originally possessing it to succeeding generations. During the process of 

 meiosis the new mutant gene may or may not be included in the gametes 

 which produce the zygotes; even if it is included in one or a few of these 

 zygotes, a series of unlucky accidents may eliminate these organisms 

 despite the high survival value of the new trait. 



300. Genetic Drift 



Professor Sewall Wright of the University of Chicago has described 

 the role of chance in the phenomenon of "genetic drift." This is the 

 tendency within small interbreeding populations for those gene pairs 

 which are heterozygous to become homozygous for one allele or the 

 other by chance, even though neither gene is particularly advantageous 

 or disadvantageous. Each small, interbreeding group thus tends to be- 

 come homozygous, genetically stable. A group which becomes stabilized 

 by chance rather than by selection is likely to have certain disadvanta- 

 geous characteristics and therefore to be eliminated subsequently. The 

 effects of genetic drift are counterbalanced in some groups by the effects 

 of mutation, selection, and occasional matings with members of another 

 group. Investigations have shown that in nature most populations of 

 animals are indeed subdivided into several or many subgroups which 

 may be small enough to be affected by genetic drift. 



The phenomenon of genetic drift, the tendency of small populations 

 to become homozygous, is an exception to the Hardy-Weinberg Law 

 (p. 682), the tendency for populations to maintain their proportions of 

 homozygous and heterozygous individuals. Since the Hardy-VV^einberg 

 Law is based on statistical events, it, like all statistical laws, does not hold 

 true for small numbers. The phenomenon of genetic drift becomes im- 

 portant in evolution whenever the effective breeding population of a 

 species becomes small, as the result, perhaps, of extreme cold, drought, a 

 severe storm or the migration of a small group to a new territory. Genetic 

 drift may help explain the common observation that similar and closely 

 related species in different parts of the world frequently differ in curious 

 ways which have no apparent adaptive value. 



301 . Preadaptation 



One of the more recent modifications of the theory of natural se- 

 lection is called the theory of preadaptation. Mutations occur completely 

 at random, and some result in characters which are either unimportant 

 or disadvantageous to the organism in its usual environment. However, 

 if the environment changes, or if the organisms migrate to a new en- 

 vironment, these same traits may be of marked value for survival. In 

 effect, an animal or plant may by chance be adapted to an environment 



