POPULATION GENETICS AND EVOLUTIONARY CHANGE 437 



tations does have a tendency to disturb any equilibrium which may exist. 

 To refer to our hamsters again, every time a gene which originally pro- 

 duced black coat color undergoes a chemical change so that it now condi- 

 tions the appearance of gray coat color, the gene pool has been altered 

 through decrease by one in number of M genes and increase by one in 

 number of m genes. Obviously, if such changes occurred frequently, con- 

 siderable modification in the ratio between numbers of M and m genes 

 might arise. But actually mutation pressure, as it is called, is observed to 

 be of low order of magnitude. Accurate data on this point are difficult to 

 obtain. In the fruit fly, Drosophila, the genetically best-known animal, it 

 is estimated that mutations of one kind or another are present in from 

 1 to 10 percent of the germ cells produced in every generation. Individual 

 genes, however, vary greatly in frequency of mutation. With some kinds 

 one out of every thousand genes, on the average, may undergo muta- 

 tion. Other kinds of genes may be so stable that only one in a billion will 

 mutate. Accordingly, there must be great variation in the efficacy of 

 mutation pressure in disturbing genetic equilibrium. Some genes may mu- 

 tate so frequently that, under particular circumstances, the constitution of 

 the gene pool is considerably altered from the equilibrium which would 

 otherwise prevail. For example, imagine a gene pool consisting of 50 per- 

 cent M genes and 50 percent m genes, and having a mutation rate such 

 that one in every thousand M genes mutates to m. It can be demon- 

 strated mathematically that in one generation the gene pool will be shifted 

 to 49.95 percent M genes and 50.05 percent m genes. This is a small change, 

 but if the same trend continued seneration after generation a considerable 

 difference in frequency of the two genes would eventually be accumu- 

 lated. Indeed, if the trend continued long enough the M genes would be 

 entirely replaced by m genes, assuming that the change from M to m 

 was unopposed. Actually the trend would be opposed by what is known 

 as reverse mutation, the mutation of m genes to form M genes. This would 

 also occur at a rather constant rate, although, judging by evidence avail- 

 able, at a rate lower than that by which M mutates to m. Thus there are 

 two opposed mutation rates: (1) the rate at which M changes to m and 

 (2) the rate at which m changes to M. The combined action of the two 

 rates is to change the gene frequency until a point is reached at which the 

 number of M genes changing to m genes in any generation balances the 

 number of m genes changing to M at that time. At this point an equilib- 

 rium is established. So we see that while mutation pressures by and of 

 themselves may alter genetic equilibriums their ultimate net effect is to es- 

 tablish equilibrium, even though it is a different equilibrium from that 

 which would otherwise prevail. (The reader is referred to Chapter 3 of 



