116 I The Process of Evolution 



distribution of the gene frequencies at a large number of loci 

 subjected to the same pressures within a single population, or as the 

 probability distribution for the chances of a given gene frequency 

 occurring in any one generation. Thus the Nm = 4 curve {ni = 4/N) 

 in Fig. 6.4 may be interpreted in the following ways. It can be said 

 that under the same given conditions the gene frequencies of a large 

 number of populations (or loci within one population) would tend 

 to cluster rather tightly around the value of .50, or that among all 

 loci within a population subjected to the same conditions the proba- 

 bility of any given locus having a gene frequency between .35 and 

 .65 is high. Finally, the curve represents the probability of the 

 gene frequency at one locus having a given value in the generation 

 observed. Thus the chance of observing a value oi q = 1 in any one 

 generation is vanishingly small. 



The type and amount of movement of genetic information ("gene 

 How") found within and among populations are important factors in 

 determining their evolution. Obviously, a situation in which a group 

 of semi-isolated subpopulations randomly exchange genetic informa- 

 tion among themselves (the "island model") is quite different from 

 a situation in which the gene How is unidirectional along a linear 

 array of subpopulations (the "river model"). In turn, both of these 

 differ from a situation in which a group of organisms is continuously 

 distributed over a large area. In the latter case, although semi- 

 discrete clusters of individuals may not exist, the probability of 

 mating by two widely separated individuals may be very low because 

 of their remoteness alone. The effects of such "isolation by distance" 

 have been dealt with mathematically by Wright, who showed that 

 the amount of local differentiation in a population is largely a func- 

 tion of the size of the panmictic units ( neighborhoods ) of which it 

 is composed. When a population is divided into semi-isolated sub- 

 populations or when some degree of inbreeding is found in the 

 population as a whole, the general result is a reduction in the fre- 

 quency of heterozygotes. An important aspect of this change in 

 genotype frequencies is illustrated by the effects in such a popula- 

 tion of selection against the homozygous recessives. This would be 

 much more effective in a subdivided or inbreeding population 

 because the reduced number of heterozygotes would "shelter" 

 fewer recessive genes. 



JOINT PRESSURES 



Up to this point, only single evolutionary forces acting on isolated 

 loci have been considered. However, in virtually all cases studied, 



