. The elements of genetics. Genetics. THE BREAKDOWN OF CONTINUITY of the mating continuum and must be met by different genetical adjustments. The mating continuum of any group in a differentiated environment therefore has an iiilierent instabihty. How is this instability resolved ? The instability arises from the exchange of genes between individuals and groups requiring different local adaptations. As fast as the local conditions select and separate more favourable combina- tions of genes, these will be broken down and brought back towards. Fig. 76.—The distribution of a recessive mutant dete
. The elements of genetics. Genetics. THE BREAKDOWN OF CONTINUITY of the mating continuum and must be met by different genetical adjustments. The mating continuum of any group in a differentiated environment therefore has an iiilierent instabihty. How is this instability resolved ? The instability arises from the exchange of genes between individuals and groups requiring different local adaptations. As fast as the local conditions select and separate more favourable combina- tions of genes, these will be broken down and brought back towards. Fig. 76.—The distribution of a recessive mutant determining "simplex" teeth in the field mouse Microtus arvalis, according to the percentage of homozygous individuals (after Zimmermann, 1937). the common level by the intrusion, from some other part of the continuum, of other combinations which have been selected under different conditions and hence adjusted in a different way. As fast as advantageous combinations of genes arise by segregation and recombination, they will break down by the same processes. The flow of variability, which arises from segregation and recombination, is necessary for the production of new and superior combinations of genes. Once, however, these combinations have been achieved, the means of their origin immediately becomes not merely urmecessary; it becomes actively harmful. The value of recom- bination lies, in fact, in bringing together combinations of 304. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Darlington, C. D. (Cyril Dean), 1903-; Mather, Kenneth. New York : Macmillan
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