THE EVOLUTIONARY CONCEPT 345 



and detailed differences that distinguish closely related populations — 

 the strains, lines, and stocks of the geneticist, and the races, subspecies, 

 and species of the taxonomist. Most of the resultant progress has been 

 due to the research methods and observations of the geneticist — experi- 

 mental breeding, cytological studies, and statistical analysis — supple- 

 mented by the accumulating results of modern research in ecology, 

 biogeography, and classification. 



We shall see that the study of speciation leads to a reaffirmation of 

 Darwinian (or neo-Darwinian) principles, as modified by our now greatly 

 clarified concepts of the limits and nature of variation, inheritance, and 

 selection, and as augmented by certain other evolutionary factors that 

 were only vaguely foreshadowed in Darwin's own writings. 



Variation as the "Raw Material" of Evolution 



As we saw in the earlier discussion of genetics (pp. 311-316), variations 

 in organisms result from two very different sets of causes. Variations of 

 one sort, often termed " modifications," are due to environmental influ- 

 ence. Those of the other sort are the result of gene or chromosome 

 differences within the organism itself. Often only breeding tests can dis- 

 tinguish between the two kinds, for they may be very similar in appear- 

 ance or in extent. The one invariable difference between them is that 

 variation produced by the environment is not heritable, while that due 

 to gene or chromosome differences is. 



If we use the term mutation in the broad sense that includes both the 

 changes of a single gene — point mutations — and the microscopically 

 visible changes in chromosome structure or complement, we can say that 

 all inheritable variations are caused by mutation. Although spontaneous 

 mutations have long been known to occur in laboratory populations of 

 animals and plants, it was not until 1927 that a means of increasing the 

 mutation rate was discovered. In that year Muller found that radiation 

 would speed up the rate at which mutations appeared, opening the possi- 

 bility for experimental study of evolutionary processes in living species. 

 It is now established that nuclei exposed to various types of radiation 

 show tremendous increases in mutation rate — increases that are roughly 

 proportional to the amount of radiation received short of that which is 

 lethal. Certain chemicals, and to a lesser degree temperature shock, will 

 also increase mutation rate. Both gene and chromosomal changes can be 

 thus induced. The gene changes can be recognized and studied by subse- 

 quent breeding tests of the radiated individuals ; the chromosome changes 

 can be seen by microscopic examination of nuclei, and their effects can 

 be studied by breeding experiments. 



It should be pointed out here that such induced mutations are in no 

 sense Lamarckian. The changes they produce are just as random and 



