MUTATION 721 



tions are changes in the arrangement of the genes on a chromosome. 

 They clearly add nothing new, but may have considerable effects 

 on the expression of a given gene. They might account for some 

 subspeciation or speciation, but not for any evolution on a wider 

 scale. Gene mutations are changes in the nature and so in the 

 effect of a gene. The rate at which they occur varies, but is seldom 

 much more than of the order of i in 10^ meioses. They allow much 

 wider scope for evolution, for we know that the different allelo- 

 morphs of a gene may have widely different effects, and if all 

 the genes in any animal mutated together the effect would be 

 large. 



ISOLATION 



We now know a good deal, from the theoretical point of view, 

 of the possible effects of selection in changing the genotype, 

 that is to say, we have mathematical formulae which connect 

 rate of spread of a gene with its mutation rate and its selective 

 advantage. Unfortunately we know very little about the magni- 

 tude of mutation rates in nature and even less about the selective 

 advantage which any naturally occurring gene may confer, so 

 that from the practical point of view the mathematics tells us 

 chiefly what we knew before — that if mutation rate and selection 

 rate are large enough evolution will occur. It has, however, 

 brought out two important points which were not previously 

 fully appreciated. The most important is that, at any rates of 

 mutation and selection which are likely, evolution in the sense 

 of splitting of one species into two, will only occur if there is 

 some degree of isolation of the two evolving populations. The 

 frequency of endemic species or subspecies on islands is thus 

 explained, for here there is often complete or nearly complete 

 isolation from the population of the mainland. Mere distance 

 may also be important, for an animal living in Spain has no 

 chance of breeding with one living in China. It is not surprising 

 therefore that over a large land mass such as Eurasia there are 

 many examples of series of subspecies or congeneric species 

 replacing each other in turn. The nuthatch {Sitta), for instance, 

 has a dozen or more forms, variously described as species or 

 subspecies, ranging from Japan to the Canaries (and there are 

 others in America) and differing chiefly in size and in whether 

 the breast is white, buff, or grey. This bird also illustrates a point 

 which is fairly general — that the Japanese and West European 



