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has attempted these adjustments can testify, but it pales in comparison to the genetic case where dozens or hundreds of interdependent alleles can be involved. Roughly, the difficulty of the problem increases by an order of magnitude for each additional gene when the interdependencies are intricate (but see the discussions in chapter 4 and pp. 160-61). |
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Given the pervasiveness of epistasis, adaptation via changes in genetic makeup becomes primarily a search for coadapted sets of allelesalleles of different genes which together significantly augment the performance of the corresponding phenotype. (In chapter 4 the concept of a coadapted set of alleles will be generalized, under the term schema,to the point where it applies to the full range of adaptive systems.) It should be clear that coadaptation depends strongly upon the environment of the phenotype. The large coadapted set of alleles which produces gills in fish augments performance only in aquatic environments. This dependence of coadaptation upon characteristics of the environment gives rise to the notion of an environmental niche,taken here to mean a set of features of the environment which can be exploited by an appropriate organization of the phenotype. (This is a broader interpretation than the usual one which limits niche to those environmental features particularly exploited by a given species.) Examples of environmental niches fitting this interpretation are: (i) an oxygen-poor, sulfur-rich environment such as is found at the bottom of ponds with large amounts of decaying mattera class of anaerobic bacteria, the thiobacilli, exploits this niche by means of a complex of enzymes enabling them to use sulfur in place of oxygen to carry out oxidation; (ii) the "bee-rich" environment exploited by the orchid Ophrys apifera which has a flower mimicking the bee closely enough to induce pollination via attempted copulation by the male bees; (iii) the environment rich in atmospheric vibrations in the frequency range of 50 to 50,000 cycles per secondthe bones of the mammalian ear are a particular adaptation of parts of the reptilian jaw which aids in the detection of these vibrations, an adaptation which clearly must be coordinated with many other adaptations, including a sophisticated information-processing network, before it can improve an organism's chances of survival. It is important to note that quite distinct coadapted sets of alleles can exploit the same environmental niche. Thus, the eye of aquatic mammals and the (functionally similar) eye of the octopus exploit the same environmental niche, but are due to coadapted sets of alleles of entirely unrelated sets of genes. |
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The various environmental niches  define different opportunities for adaptation open to the genetic system. To exploit these opportunities the genetic system must select and use the sets of coadapted alleles which produce the appropriate phenotypic characteristics. The central question for genetic systems is: How |
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