214 I The Process of Evolution 



size. Clausen, Keck, and Hiesey concluded from transplant studies 

 along a 200-mile transect of California that the genotypes and local 

 populations are arranged into at least 11 physiological races. Two 

 taxonomic species are represented along this transect, where they 

 occur in different habitats. Achillea lanulosa, of the higher eleva- 

 tions, is primarily a species of continental habitats, whereas A. 

 borealis occurs at lower altitudes and is a coastal species, in the 

 main. It is interesting to note that in the northern portion of its 

 range, where A. lanulosa comes to the coast, it has developed coastal 

 ecotypes that mimic those of A. borealis. 



It may well be that plants, being rooted, become adjusted to the 

 local conditions with a precision that would not be of selective value 

 in animals. 



Clinal Variation in Plants. The butterfly weed (Asclepias tuhe- 

 rosa) also shows geographic variation, but it has been studied in a 

 very different manner. The subspecies occurring in the eastern two- 

 thirds of the United States have been studied in great detail by 

 Woodson. The distribution of these subspecies is shown in Fig. 

 10.1. Only A. tuberosa tuberosa and A. t. interior will be discussed 

 here. In most parts of their range these subspecies can be dis- 

 tinguished by the shape of the leaf. Fortunately, two important 

 components of leaf shape can be quantified and the change in 

 shape studied geographically. These components are angle A, a 

 measure of the taper of the apex of the leaf, and angle B, which 

 measures the shape of the base of the leaf. The two subspecies meet 

 along a broad front in the eastern United States, and there is a zone 

 of intergradation, as can be seen in Fig. 10.1. Woodson has studied 

 geographic variation by dividing a map of the country into equal- 

 area quadrats and measuring the herbarium specimens collected 

 in these areas. He has also studied local-population samples and 

 has measures of variation within and between individuals and 

 colonies. 



By comparing the measurements of specimens collected in 1946 

 along a 1,200-mile transect from Kansas to Virginia with the available 

 herbarium specimens from the quadrats in which the transect falls, 

 Woodson was able to study the effect of time. The herbarium speci- 

 mens, collected over a period of many years, represent a sample 

 which is, on an average, older than the 1946 transect. It was clear 

 that characteristics of A. t. interior were moving eastward, while 

 those of A. t. tuberosa were moving westward but at a much slower 

 rate. When, in 1960, samples were once again collected along the 

 transect, the changes that had occurred in the 14-year interval could 

 be accurately measured. Apparently reciprocal diffusion of the 



