EVOLUTION OF THE GENIC 

 MATERIAL 



Most of the facts pertaining to this chapter have akeady been 

 mentioned in connection with the theory of the gene. The first group 

 we shall consider here deals with the evolution of the chromosome 

 set. It is one of the surprising features of cytogenetics that the 

 chromosomal cycle and the karyotype have not changed much in the 

 phylogenetic tree. Some chromosomes of Protozoa have the same size, 

 form, number, and finer structure as those of the highest organisms, 

 for example, Monocystis ( gregarines ) and the large symbiontic flagel- 

 lates. If we accept Moewus' work on the linkage groups in Chlamy- 

 domonas, the genetic structure of chromosomes in Protista — not to 

 mention bacteria — closely parallels that of higher animals and plants. 

 If we assume that the chromosome has been "invented" to make 

 possible the exact duplication of the genie material (Roux, 1883), the 

 lack of visible evolutionary changes in this mechanism indicates that 

 whatever evolutionary changes took place within the chromosome, 

 they were changes that did not affect the visible features of the 

 chromosome. 



Within this constancy of the general features of the chromosomal 

 apparatus, some diversification has taken place for which it is very 

 difficult to find an explanation. If we say, as has been done, that the 

 chromosomal configuration (the karyotype) is just a morphological 

 character like all others, we do not gain much insight. Other morpho- 

 logical characters are the result of mutation and selection in some 

 way. But how selection could produce the characteristic karyotype of 

 sauropsids or of most Lepidoptera is a question which seems un- 

 answerable. A number of larger or smaller taxonomic groups have 

 karyotypes so characteristic that the group could be diagnosed from 

 a metaphase plate. All birds and reptiles have a circle of large chromo- 

 somes with a number of small ones in the center; and it is most 

 suggestive that the Monotremata have the same karyotype (see 

 Matthey, 1949). The majority of Lepidoptera have a group of about 

 60 dotlike chromosomes arranged in a very characteristic way. Many 

 urodeles have about 24 very long, looplike chromosomes. Many Or- 

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