THE ORIGIN OF VARIATION 



Subspecies need not occur in clines. That is, if a series of subspecies, 

 A, B, C, D, and E occur in that order over the geographic range of the 

 species, A might resemble B most closely in size, while it resembles D 

 most closely in color. In a third character, it might resemble C most 

 closely, and so on. In many cases, subspecies can be defined only by a 

 statistical analysis of several variable characters in the various populations 

 of a species. Subspecies of North Sea herring, for example, have been de- 

 fined by means of statistical studies of such characters as number of ver- 

 tebrae, number of scales in the lateral line, number of rows of scales 

 above and below the lateral line, and many other variable characters. The 

 resulting statistics show that the averages and standard deviations for 

 each school are typical, and differ from all others, with the result that 

 they may be described as subspecies. 



Now Darwin had already spoken of subspecies as "incipient species 

 . , . ( which ) . . . become ultimately converted into good and distinct spe- 

 cies . . . by . . . natural selection." As already mentioned, Darwin realized 

 that the weakest point in his theory was the lack of knowledge of heredity. 

 The dominant school of evolutionary thinking today is based upon the 

 Darwinian principles of the prodigality of nature, variation, and natural 

 selection. But to this is added the interpretation of variation in terms of 

 the theory of the gene, and the study of populations and changes in gene 

 frequency with the tools of statistics which, like genetics, did not exist 

 in Darwin's time. This school is therefore known as the neo-Darwinian 

 school, and it is exemplified by most of the outstanding students of evolu- 

 tion of today, as Sewall Wright, Th. Dobzhansky, G. L. Stebbins, and 

 Julian Huxley. 



For the neo-Darwinians, then, subspecies are, at least potentially, in- 

 cipient species. Each subspecies of a particular species is characterized 

 by a particular complex of gene-determined characters, and these various 

 genes are derived by mutation from an originally more uniform progeni- 

 tor. The genes which any particular subspecies possesses are not peculiar 

 to it, but are found in varying percentages in different subspecies. It is 

 the constellation of genes which is unique for each subspecies, not the 

 individual genes. Such gene combinations are the materials upon which 

 selection acts. Within such subspecies, mutation continues at random, 

 with the result that different alleles arise in different subspecies. Now 

 subspecies are usually at least partially isolated from each other geograph- 

 ically. Other types of isolation (to be discussed below) may also arise, 

 for example physiological or ecological isolation. Within such isolated 

 subspecies, new mutations, as they arise, cannot spread throughout the 

 species. Thus, by the gradual accumulation of mutations, such isolated 

 subspecies may "become ultimately good and distinct species." The spec- 

 trum of mutation extends from gross defects such as lethality or crippling 

 through moderate effects like color change or change in bodily propor- 

 tions to very minor changes detectable only by special methods. Neo- 

 Darwinians generally favor the latter, for several reasons. First, most of 

 the larger mutations are plainly abnormalities which would be eliminated 

 by natural selection. This may be a consequence of the fact that the more 



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