GENE MUTATION 



several subspecies of any one species do inhabit ranges which are mutu- 

 ally exclusive for the most part. Closely related species may have identical 

 ranges, but subspecies replace one another geographically. The whole 

 series of geographic subspecies for any species is called a Rassenkreis 

 ( German— circle of races ) . 



Wherever subspecies are distributed over an area which presents a 

 progressive change in some physical feature, such as mean annual tem- 

 perature, some of the differences in the subspecies are likely to show a 

 progressive change also. Such series are called dines. One can scarcely 

 doubt that such characters, which vary progressively along with a pro- 

 gressive change in climate or topography, must be of adaptive value to 

 the various subspecies concerned. Thus, Alpatov found that the average 

 size of bees increased progressively from southern to northern Europe, 

 while the lengths of legs and tongue decreased along the same cline. 

 Similarly, Turesson has shown that plants commonly show definite char- 

 acter complexes according to the type of habitat in which they live. Thus 

 in Primula (primrose) there are ecotypes adapted to alpine habitats and 

 there are ecotypes adapted to meadowlands, and to as many more habitat 

 types as the plant is naturally found in ( Figure 75 ) . Here again, one can 

 scarcely doubt the adaptational value of a character complex which is 

 always found in plants which live in a particular type of environment. 

 Wherever such ecotypes form a cline, they coincide with the subspecies. 

 Sometimes factors leading to such cline formation can be summarized in 

 concise rules. Thus Bergmann's rule states that, in a given species or 

 genus, northern populations have a larger mean body size than do south- 

 ern populations. This is believed to depend upon the fact that a large 

 body has a smaller surface area per unit volume than does a small body. 

 Thus conformity to the rule promotes heat conservation in the north and 

 heat conduction in the south. A corollary to this is Allen's rule, which 

 states that extremities (hmbs and tails) tend to be smaller in the north 

 than in the south. Again, this is interpreted in terms of restriction (north) 

 or extension (south) of radiating surfaces. Scholander has recently ex- 

 plained adaptation on quite a different basis. Arctic mammals have very 

 effective insulation of fur or feathers, as well as circulatory control of heat 

 loss, which permits them to maintain the body temperature against severe 

 cold by basal metabolism alone. The less protected extremities are physi- 

 ologically adapted to function well even when thoroughly chilled. He 

 believes that these findings invalidate Bergmann's and Allen's rules. Still, 

 the number of analyzed cases in which the rules appear to apply is so 

 great, and their explanation on any other basis would be so difficult, that 

 zoologists generally have not been convinced of Scholander's thesis. Prob- 

 ably the physiological adaptations are of major importance, yet under 

 conditions of severe competition even the small additional advantage 

 gained by size variation in accordance with these rules is sufficient to give 

 a selective advantage to those populations which conform. Gloger's rule 

 states that animals in cool, dry regions tend to be lighter in color than 

 those in warm, humid regions. Exceptions to these rules can be found, 

 but they have considerable validity. 



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