42 evolution: the ages and tomorrow 



bound to appear. Mutant and chromosome changes arise, 

 selection acts, and evolution is on the move. The effective- 

 ness of the adaptational selective factors and mutational 

 change will be most fully realized in a population of just the 

 right proportions. The ideal seems to be a fairly large pop- 

 ulation which is broken up into smaller breeding groups 

 with some interchange between groups by cross-breeding. 

 This is the common case. Very small populations endanger 

 the organism and selection may be ineffective due to the 

 purely random nature of the mutant changes. This, in a 

 manner of speaking, is the crux of the whole population ex- 

 planation—a group must be large enough to show gene drift 

 by differential reproduction. Isolation, geographic or other- 

 wise, is a potent factor in speeding up evolution, provided 

 the isolated groups do not remain too small. 



Various sizes and situations have been analyzed in this 

 new genetics of population, and many of the inadaptive and 

 nonadaptive puzzles of evolution have been clarified. Add 

 to these statistical studies the possibility that the usual small 

 mutations (micromutations) are sometimes aided by sudden 

 large changes (macromutations) which appear at times in 

 the species and, all things considered, we have a basic and 

 flexible explanation of the evolutionary record. Indeed, it 

 is not an exaggeration to say that the main evolutionary 

 problem has been solved; only the details remain to be 

 worked out, details such as those which T. Dobzhansky 

 reviews in his Genetics and the Origin of Species. 



Throughout all these studies the observation is commonly 

 made that the vast forces operating in evolution work both 

 for and against the organism. Obviously nature does not 

 control the trends in any idealistic manner. Mutation can 

 become rampant and literally destroy the organism through 

 the accumulation of undesirable characteristics; or, as a con- 

 verse, the heredity may be so inflexible as to destroy the 

 organism when environmental conditions change. This lat- 

 ter has probably been a common cause of extinction. Failure 



