SELECTION AND VARIABILITY 



Tradescantia virginiana, the iicwt Triton pahnatus and, as we saw, in 

 the species of Oenothera. Localization of cither kind of course, does 

 something more than merely reduce recombination. It divides the 

 genes of each chromosome into two regions, one with high and 

 the other with low recombination or none at all. It is interesting, 

 if disappointing, to discover that although the heterochromatin falls 

 in the low group in Paris it occurs in Fritillaria chiefly in species 

 without localization and in regions where crossing-over occurs. 



The third method of reducing recombination is found only in 

 animals. It consists in nothing less than the abolition of crossing-over 

 in the heterozygous sex. A new method of holding the chromosomes 

 together at meiosis is found in the males of certain Diptera and 

 Orthoptcra. The attraction between chromatids is extended from 

 twos to fours, and the early repulsion of centromeres is suppressed. 

 Crossing-over is retained with a normal meiosis in the homozygous 

 sex. This means that a chromosome passing, for example, from one 

 male Drosophila to another may avoid crossing-over generation 

 after generation. But this may also happen with an ordinary low 

 frequency of crossing-over in plants. For, with a single chiasma, 

 only two of the chromatids are cross-overs, and from each bivalent 

 two of the four germ cells have unchanged chromosomes. So far as 

 the species is concerned, therefore, the genetical result of the abolition 

 of crossing-over in one sex is quite indistinguishable from a 

 reduction of chiasma-frequency to one half in both sexes, itself an 

 unworkable arrangement since, as we saw, that frequency is usually 

 already at a working minimum. 



These variations show that, in the development of the genetic 

 systems of plants and animals, natural selection has paid a great 

 deal of attention to this problem of recombination. Close linkage is 

 nearly always encouraged. This we can understand if the tendency 

 is for variability to exist in the form of balanced linked combina- 

 tions, genetically unlike but of similar effect on the phenotype. In 

 such cases, the amount of recombination favoured over any period 

 will depend on the balance of advantage of fitness and flexibility, 

 and hence indirectly on the rate of change of the environment during 

 that period. Since only occasional recombination is required to 

 maintain flexibility, the frequency of crossing-over which is 

 favoured will generally be low. 



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