NATURAL SELECTION: II 503 



studies can, with caution, be applied to explanation of origin of the larger 

 diflferences between families, orders, classes, and phyla. The large evolu- 

 tionary changes, connected as we have seen with major changes of en- 

 vironment, constitute the most important accomplishments of evolution. 

 Accordingly, brief consideration of mega-evolution will form a fitting cli- 

 max to our discussion. 



Evidence accumulates that extent of evolutionary change and rapidity 

 of evolutionary change are connected. So far as we can judge from the 

 geologic record, large changes seem usually to have arisen rather suddenly, 

 in terms of geologic time. This fact has been one of the reasons why a spe- 

 cial type of large mutation, "systemic mutation," has been postulated by 

 Goldschmidt (1940) as necessary to account for the large changes observed 

 in evolution. By "systemic mutation" Goldschmidt meant a complete 

 repatterning of the chromosomes — "the arrangement of the serial chemical 

 constituents of the chromosomes into a new, spatially different order; i.e., 

 a new chromosomal pattern." Chromosomal aberrations (pp. 396-400) 

 quahfy as systemic mutations under this definition. But are chromosomal 

 aberrations of great importance in producing major evolutionary change? 

 While we have seen that new species of plants may arise suddenly by 

 polyploidy (pp. 418-420), it is noteworthy that chromosomal aberrations 

 frequently produce less marked effects upon their possessors than do gene 

 mutations. In other words, the arrangement of the genes present usually 

 makes less difference than does the nature of the genes present, whatever 

 their arrangement in the chromosomes. Hence most students of the sub- 

 ject regard gene mutations as of more importance to evolution than are 

 those "systemic mutations" known to us (i.e., chromosomal aberrations). 



What explanation other than that of "systemic mutations" can we find for 

 rapid occurrence of large changes? As noted before, rapid change may be 

 expected to occur when an organism faces the demands of a new environ- 

 mental niche radically different from the one formerly occupied. Under 

 such conditions the severity of natural selection will be greatly increased, 

 with resultant increase in rate of evolution. We recall recent experimental 

 evidence (pp. 460-464) indicating that natural selection can, upon occa- 

 sion, operate with surprising swiftness. Organisms faced with radically new 

 conditions of life will adapt rapidly to those conditions under the stimulus 

 of a severe natural selection operating upon the raw materials provided in 

 the form of mutations and other types of genetic variability. We recall, 

 also, that such factors as differential growth rates may magnify the effects 

 produced by single mutations. 



Another pertinent observation is the fact that fossil forms intermediate 



