The Differentiation of Populations j 237 



marine organisms, previously more or less continuously distributed, 

 become isolated. There is considerable evidence that glacial ad- 

 vances during the Pleistocene repeatedly fragmented the ranges of 

 many organisms. Repeated changes of continental seaways (Fig. 

 10.13) have isolated and reconnected portions of the continents, 

 causing manifold changes in the continuity of the distributions of 

 organisms. 



Climatic changes have profound effects on the distributions of 

 plants and animals. Trends toward aridity produce desert or steppe 

 barriers to the passage of organisms requiring high humidity or 

 dense plant cover. Drought may cause large lakes to divide into 

 numerous smaller lakes and rivers to be reduced to isolated series of 

 pools. Increasing rainfall, on the other hand, tends to reunite iso- 

 lated bodies of water, encouraging gene exchange in aquatic or- 

 ganisms while forming barriers for terrestrial organisms. Belts of 

 high humidity form barriers for desert and steppe organisms. 



It is interesting to note that discontinuities in the distribution of 

 an animal are not necessarily the result of barriers which the indi- 

 viduals are unable to cross. In many cases behavior patterns prevent 

 dispersal across areas that could easily be traversed if the attempt 

 were made. Thus rivers may serve to isolate bird populations on 

 opposite banks, or a narrow strip of woods may effectively separate 

 two meadow populations of butterflies. 



Isolation 



Whatever its cause, physical isolation permits the differentiation of 

 populations of sexually reproducing organisms. Isolation is often 

 referred to as if it were the cause of differentiation. It is not, of 

 course; recombination between isolates is prevented, so that each 

 isolate responds only to the selection pressures of its own environ- 

 ment. If an organism is continuously distributed along a humidity 

 gradient, the establishment of populations at the ends, which are 

 adjusted to high and low humidity, will be hindered by the transfer 

 of genetic information back and forth along the gradient. If the 

 middle of the continuum is destroyed, differentiation can continue 

 without such interference. By definition, the environments in which 

 two newly isolated fragments of a population find themselves cannot 

 be identical. Thus the two isolates are subjected to different selection 

 pressures. Because of sampling error, the two new isolates will be of 

 different sizes and will have gene pools that are not identical, so 

 that evolutionary forces will be operating in unlike genetic environ- 

 ments. Since mutation is a random process, it is not to be expected 

 that identical mutations will show up in the two isolates. Thus, 



