448 INTRODUCTION TO EVOLUTION 



ble. The number of individuals of this species fluctuates between about 50 

 and 400. At times the minimum efi'ective breeding population may thus fall 

 below 50 individuals. No doubt the Sewall Wright effect has been a very 

 significant factor in the evolution of this remarkable species, which is one 

 of the most distinctive in the genus." (We suffer from a plethora of terms; 

 "random variation of gene frequency," "genetic drift," "random survival," 

 "Sewall Wright effect" all refer to the same phenomenon.) This example 

 is an extreme one in that evolution has proceeded so far that the inhab- 

 itants of this spring are regarded as constituting a distinct species. It is not 

 certain that the effective breeding population is abnormally small, how- 

 ever. We should note in this connection that the number of individuals 

 of importance to genetic drift is not the total number in the population 

 but only the portion of the latter which actually contribute genes to the 

 next generation. Immature, sterile, aged, and infirm individuals are not 

 part of the "effective breeding population." Furthermore, in many species 

 the effective breeding population is restricted in size during certain seasons 

 of the year, particularly, in the Temperate Zone, during the winter. It is 

 this minimum size of effective breeding population which is important in 

 determining the occurrence of random variations in gene frequency. 



Elementary students are frequently dismayed by what seem to them 

 flat contradictions, such as those contained in this discussion of genetic 

 drift. We should realize, however, that such differences of opinion are in- 

 evitable in a growing science. It behooves us not to be dogmatic in the 

 present state of knowledge. We know too little about the genetic constitu- 

 tions of animals and the varied effects of single genes, about the size of 

 effective breeding populations actually found in nature, about the effec- 

 tiveness and action of barriers, and particularly about the relationships of 

 animals to their environments, i.e., the ecology of animals. Evolution does 

 not occur in a vacuum. The animal is an integral part of a community of 

 plants and animals played upon by a variety of inanimate environmental 

 factors: temperature, humidity, climate, seasonal changes, nature of the 

 soil, and so on. Until all these animate and inanimate factors in the en- 

 vironment of an animal are understood we cannot have a complete pic- 

 ture of the demands-of-life faced by that animal, and hence of the oppor- 

 tunities for operation of natural selection. We have intimated (p. 324) that 

 in the case of wood lots three miles apart it is difficult to imagine en- 

 vironmental differences sufficient to give rise, through natural selection, 

 to the observed differences in mouse inhabitants. That is true, but there 

 are many environmental factors involved which are yet unanalyzed. 

 Also, recent experiments in natural selection (pp. 460-464), give evi- 



