274 INTRODUCTION TO EVOLUTION 



continuous, related forms being found in regions between the center of 

 dispersal and outlying areas. Such a situation is no less interesting for the 

 study of evolution than is discontinuity of range. In fact, in his brief auto- 

 biography, Darwin mentioned this fact of "the manner in which closely al- 

 lied animals replace one another in proceeding southwards over the 

 Continent" (South America) as one of the three observations which most 

 strongly directed his thinking into evolutionary lines. 



In his Origin of Species Darwin wrote, "The naturalist, in traveling, for 

 instance, from north to south, never fails to be struck by the manner in 

 which successive groups of beings, specifically distinct, though nearly re- 

 lated, replace each other. He hears from closely allied, yet distinct kinds 

 of birds notes nearly similar, and sees their nests similarly constructed, but 

 not quite alike, with eggs colored in nearly the same manner." 



This observation of Darwin's suggests a frequently observed phenome- 

 non: that related races or species may be observed to vary in a regular 

 way as one progresses from one part of their geographic range to another. 

 For example, in the common zebra inhabiting the southern half of Africa 

 the black striping of the legs steadily decreases (and in a form now extinct 

 eventually disappeared) in a progressive series from north to south (Fig. 

 12.6). Such a character gradient has been called by Julian Huxley a dine. 

 Multitudes of examples might be cited (for comprehensive summaries see 

 Goldschmidt, 1940, and Huxley, 1942). A typical example comes from the 

 work of Alpatov, who investigated honeybees throughout European 

 Russia. He found that from north to south the tongue increases in length, 

 the abdomen becomes lighter in color, and the wax gland decreases in 

 size (Goldschmidt, 1940). These changes were observed to occur in a regu- 

 lar gradient, intermediate localities having bees with intermediate char- 

 acteristics. In this case at least some of the changes observed may be 

 connected with adaptation of the bees to different conditions. The nectar- 

 secreting flowers in southern Russia may differ from those in northern Rus- 

 sia sufficiently to necessitate the longer tongue observed, for instance. We 

 might well expect that when over a wide territory the climate and other 

 environmental factors change gradually, the nature of the organisms in- 

 habiting the successive parts of the territory would change gradually also. 

 This is not to suggest that all characteristics observed to form clines are 

 adaptive in nature. Some may be neutral (nonadaptive). In the absence 

 of any evidence to the contrary, for example, we might suspect that the 

 striping of the legs of the zebras noted above is such a neutral trait. Such 

 phenomena as the gradual dispersion of genes from a center of dispersal 

 could give rise to chnes even when the resulting variation in characteristics 



