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genes). In this way one operon can cause the cell to produce signals which (with controlled delays) turn on other operons. This provision for action conditional upon previous (conditional) actions gives the chromosome tremendous information-processing power. In fact, as will be shown in chapter 8, any effectively describable information-processing program can be produced in this way. |
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For the geneticist, the picture of the process of adaptation which emerges from the mathematical treatment thus far exhibits certain familiar landmarks: |
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Natural selection directs evolution not by accepting or rejecting mutations as they occur, but by sorting new adaptive combinations out of a gene pool of variability which has been built up through the combined action of mutation, gene recombination, and selection over many generations. For the most part Darwin's concept of descent with modification fits in with our modern concept of interaction between evolutionary processes, because each new adaptive combination is a modification of an adaptation to a previous environment. (p. 31) |
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Inversions and translocations of chromosomal segments, when present in the heterozygous condition, can increase genetic linkage and so bind together adaptive gene combinations. . . . The importance of such increased linkage is due to the number of diverse genes which must contribute to any adaptive mechanism in a higher plant or animal. (p. 57) |
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Stebbins in Processes of Organic Evolution |
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Not only do we claim in this case [of inversions found in D. pseudoobscura and D. persimilis]that the precise pairing of the chromosomes in the species hybrids shows that the chromosomal material has had a common source, but we also claim that the sequence of rearrangements [produced by inversions] that occurred in the chromosome reconstructs for us the precise pattern of change that led up to and then beyond the point of speciation. |
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Wallace in Chromosomes, Giant Molecules, and Evolution (p. 49) |
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At the same time the emphasis on gene interaction poses a series of difficult problems: |
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Intricate adaptations, involving a great complexity of genetic substitutions to render them efficient would only be established, or even maintained in the species, by the agency of selective forces, the intensity of which may be thought of broadly, as proportional to their complexity. |
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Fisher in Evolution as a Process,ed. Huxley et al. (p. 117) |
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