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The Mystery of the Genome
nucleotides. So within any limited gene sequence there is essentially no recombination. Any such block of DNA that does not have recombination is subject to “Muller’s ratchet” (Muller, 1964). This means that the good mutations and the bad mutations cannot be separated. Since we know that the bad mutations overwhelmingly outnumber the good, we can be certain that any such stretch of DNA must degenerate. The hordes of bad mutations will always drag the rare good mutations down with them. While we are waiting for a rare beneficial mutation, bad mutations are piling up throughout the region. Even if we could succeed in accumulating perhaps a hundred “good” mutations within a region, and were waiting for the next one to come along - we would start to see many of our good mutations start to back-mutate into the bad. Time is our enemy in this situation - the more time, the less information. Muller’s ratchet will kill a new gene long before it can take shape. b) Too much selective cost - In previous chapters we have discussed the cost of selection. Haldane’s dilemma only considers progressive selection. But we can only afford to “fund” progressive selection for beneficial mutations after we have paid for all other reproductive costs - including all costs associated with eliminating bad mutations. As we have already seen, there are so many bad mutations we cannot afford even to pay just the reproductive cost of eliminating them. Since we cannot afford to stop degeneration - we obviously have nothing left over to fund progressive selection. There is just one way around this. In the short run, we can fund progressive selection for a very limited number of traits - if we borrow “selection dollars” from our long-term struggle against bad mutations. However, we need to understand that this means that any short-term progress in terms of specific beneficial mutations, is paid for by faster genomic degeneration in the long run. c) Non-random mutation - As it turns out, mutations are not entirely random. Can this help us to create new genes? No, it makes our problem much worse! For example, we