252 I The Process of Evolution 



our common blues and hairstreaks ) , but its many structural pe- 

 culiarities clearly set it apart from other lycaenids, and it is placed 

 in a separate subfamily. Again, nothing is known of the degree of 

 differentiation that may exist within the cluster. 



If one ignores his fossil record, man is a very distinct organism. 

 There is complete reproductive isolation between Homo sapiens 

 and his nearest living relatives (the anthropoid apes). As far as is 

 known, differentiation within the human species has not progressed 

 to the point where segregates within the species are infertile upon 

 crossing. However, in contrast to Sphenodon and Styx, there is a 

 great deal of geographic variation within Homo sapiens. Our con- 

 cepts of "race" are based primarily on variation in a few conspicu- 

 ous external characters (skin color, hair type, skull shape), but there 

 is also variation in less obvious characters, notably blood type and 

 hemoglobin type (Chap. 7). The Homo sapiens cluster therefore 

 does not fragment easily into distinct subclusters. Virtually all sub- 

 groups of man exchange genes to some extent, with the result that 

 patterns of variation are exceedingly complex. Indeed, discordant 

 variation in which patterns for the various characters studied are 

 widely different is very common. A good example of this discordant 

 variation can be seen in the comparison of distributions of blood 

 types in human populations (Fig. 11.1). The distributions of blood 

 group genes A and B show little resemblance. 



Examples of very distinct forms are also found commonly in the 

 plant kingdom. Ginkgo biloba, the maidenhair tree, is, like Spheno- 

 don, the only living member of its group, a very distinct order of 

 gymnosperms. The phyletic line to which it belongs can be traced 

 far back into the fossil record. During the Mesozoic there were 

 many genera and species of Ginkgoales (Fig. 11.2). For some rea- 

 son unknown to us, the line became extinct in the Tertiary, with 

 the exception of Ginkgo biloba. The exact native habitat of the 

 ginkgo is not known, but for centuries the Chinese have cultivated 

 the species for its edible seeds. In China most of the plants are 

 grown in temple gardens. Since the plant has virtually no insect or 

 fungal pests and since its leaves turn a striking gold color in autumn, 

 ginkgo has become a common and popular street tree in most 

 temperate parts of the world. It seems to survive in areas where the 

 worst environmental pollutions which man can produce are abun- 

 dant. Ginkgo, like many other isolated gymnosperms, is very diverse 

 genetically, having many horticultural variants. 



The dawn redwood, Metasequoia glyptostroboides, also appar- 

 ently is the last survivor of a genus of gymnosperms related to 

 Sequoia and Sequoiadendron. Unlike Ginkgo, Metasequoia was first 



