POPULATION GENETICS AND EVOLUTIONARY CHANGE 439 



We mention this matter to emphasize the fact that recessive mutations 

 may occur and be carried by a population as a sort of hidden store, with 

 only occasional homozygous recessive individuals being produced to dis- 

 play the phenotypic effect of the gene. Yet this time lag between the 

 occurrence of the new mutation and the appearance of an individual show- 

 ing its phenotypic eflfect occurs only when the new mutation is completely 

 recessive. If a normally occurring recessive gene mutates to form a domi- 

 nant gene (e.g., tn to M), a visible eflfect is produced in first-generation 

 oflFspring. Here we may recall that most European hamsters are gray, 

 hence the "black gene" must have arisen in just this way. 



In an earlier discussion (pp. 346-347) we noted that frequently muta- 

 tions classed as recessive are not completely so — that they produce some ef- 

 fect in heterozygotes even though the dominant member of the pair may 

 produce the principal eflfect. When this is the case, an eflfect of the "reces- 

 sive" mutation will be manifested in the first generation following the 

 change in the gene. As we noted, evidence is accumulating that frequently, 

 perhaps usually, it is the heterozygous state rather than the homozygous 

 one that is important in evolution. 



GENETIC DRIFT 



Let us return in imagination to the population of black 

 hamsters all of which are MM except for the one Mm individual. We saw 

 that the only mating possible for the latter is to an MM individual. We 

 then stated that such a mating (Mm X MM) is expected to produce oflf- 

 spring that are half Mm, half MM. Now that statement is true on the av- 

 erage, but such theoretically expected ratios are merely expressions of the 

 operation of the laws of probability, as in flipping coins. If parental pairs 

 of MM X Mm constitution give rise to 200 oflfspring, about 100 of them 

 would be MM, about 100 Mm. But let us suppose that this single pair of 

 parents had only two oflfspring — at least, two which lived to reproduce in 

 their turn. Such a situation commonly prevails in populations that are sta- 

 tionary in numbers from generation to generation. If the parents produce 

 only two oflfspring which live to reproduce, what eflfect will that fact have 

 upon the fate of the mutation, m, in which we are interested? 



MM X Mm 



M -^ A/1 



germ 

 cells 



M^m 



cells 



