The Differentiation of Populations | 233 



parable eastern genotypes. In A. tuberosa interior the genotype for 

 Z B has a selective value about three times that for Z A. During 

 the 14 years there has been a decrease of about 30 percent in vari- 

 ability for both Z A and Z B. 



In both years, about midway along the transect there appeared to 

 be a zone of heterozygosity expressed as greater variability as well 

 as greater size and apparent vigor. It is probable that hybridization 

 between these subspecies, which were separated from one another 

 during the Pleistocene glaciation and which have subsequently 

 come together, has resulted in an increase in vigor and an extension 

 of range. This study is one of the few in which populations have 

 been investigated over a period of years. Unfortunately, however, 

 Asclepias tuberosa is not a good subject for genetic studies. The 

 cytogenetic bases for the phenomena that have been detected bio- 



Fig. 10.10 I Egg mimicry. Upper row, eggs of Asiatic crows, Coruus 

 coronoides, C. splendens insolens, and C. s. protegatus; lower row, eggs of 

 the cuckoo Eudynamys scolopacea laid in the same nests as the three 

 crow eggs. {After Baker, 1923, Proa. Zool. Soc. Lond.) 



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