The Differentiation of Populations j 213 



In many cases, variation is not in frequency of types (partially 

 intrapopulational ) . Individuals within a given population may all 

 be closely similar (little intrapopulational variation), but color or 

 pattern may change from population to population in broad geo- 

 graphic trends. Among mammals and birds the tendency for popu- 

 lations in colder, drier parts of the range to be hghter than those in 

 the warmer, more moist parts is so common as to have been dignified 

 as Gloger's rule. Other so-called "ecological rules" deal with varia- 

 tion (not necessarily continuous) in size and shape. One (Berg- 

 mann's ) states that homoiothermal vertebrates in warm areas tend to 

 be smaller than those from cool areas. Another (Allen's) states that 

 all projecting parts (wings, legs, noses, etc.) tend to be shorter in 

 cooler areas than in warm ones. 



Ecotypic Variation in Plants. Botanists have attached more im- 

 portance than have zoologists to the local population as a basic unit, 

 perhaps because of the greater ease with which the less motile plants 

 may be studied. The work of Clausen, Keck, and Hiesey over many 

 years has been directed to an analysis of the variation within and 

 between populations of plants widely distributed in California. Mak- 

 ing use of field growing stations at Stanford (sea level), Mather 

 (4,600 feet), and Timberline (10,000 feet), they have been able to 

 separate, to a large extent, environmental and genetic components of 

 variation. Perennial plants that can be propagated vegetatively may 

 be grown at all these locations and their physiological responses to 

 environmental factors thus investigated. In effect, the same genetic 

 individual mav be studied simultaneously in three different eco- 

 logical situations. Studies such as these have led to recognition of 

 the ecological race or ecotype of plants. 



The genus Achillea (yarrow) in the sunflower family has already 

 been mentioned. By means of transplant studies, Clausen, Keck, 

 and Hiesey have analyzed the A. millefolium complex in some detail. 

 The plants are found throughout the Northern Hemisphere, where 

 they grow from sea level to timberline. There is continuous morpho- 

 logical variation, from plants some 6 feet high in the San Joaquin 

 Valley to alpine plants only a few inches in height. Other morpho- 

 logical traits also intergrade from population to population, so that 

 taxonomic distinctions are difficult to determine. 



Adjustment of Achillea plants to their environment depends on the 

 proper integration of many physiological processes, such as rates of 

 photosynthesis and respiration, resistance to cold, and time of dor- 

 mancy and other periodic phenomena. Each local population is 

 composed of many different genotypes. Depending upon the level 

 of study, these can be viewed as aggregated into groups of varying 



