The mutation rate Essay Example for Free

The mutation rate Essay One of the greatest challenges for evolutionary biology is explaining the widespread occurrence of sexual reproduction, and the associated process of genetic recombination. Sexual reproduction involves one individual combining half its DNA with half of DNA of another individual, so that the offspring is only half genetic copy of each parent. However, in asexual reproduction, the offspring are genetic copies of the parent. Thus, sexual reproduction poses an evolutionary problem because it seems to be half as efficient a method of reproducing as asexual reproduction. Asexual females can potentially produce twice as many daughters as sexual females, so that the ratio of asexual to sexual females should initially double each generation, resulting in the two-fold cost of sex.? In addition to this 50% cost and the dilution of the individuals genome, sexual reproduction also presents other disadvantages in comparison to asexual reproduction. First and foremost there is the cost of recombination favourable gene combinations that have increased in frequency under the action of natural selection are broken up. Secondly, the process of sexual reproduction requires a significant cellular-mechanical cost as sex requires meiosis, syngamy and karyogamy. A great amount of time is taken up by these three processes alone, and far exceeds the time required for two mitotic divisions. Also, these processes are unnecessarily complicated if reproduction is sole objective. Asexual spores and meiosis-bypassing apomixis appears much more efficient. Thirdly, sexual species can not perpetuate what are often fitness-improving types (eg: triploids, aneuploids). Finally, the actual physical contact between organisms/gametes entails risks separate from those that are maintained by sexual competition. One of the primary costs of initial contact is the risk of disease or parasite transmission. Another significant cost associated with fertilisation is the eminent wastage of gametes, or more appropriately in some cases, a waste of effort transmitting gametes. Given all of these costs, we would expect natural selection to favour asexual reproduction in wild populations, however, it generally does not: sexual reproduction is widespread throughout the animal and plant kingdoms. Sexual reproduction must enjoy some evolutionary advantage, which means that the advantage is not caused by the process itself, but by the changes it causes in progeny genotypes (as a result of recombination), which should drive the evolution of sex. Thus, the problem of explaining sex is to find a compensating advantage of sexual reproduction that is large enough to make up for its cost. Many theoretical models have been developed to show the conditions under which there is a sufficiently large short-term advantage for sex to offset this two-fold cost. The general consensus amongst evolutionary biologists is that there are two relatively convincing, modern day theories. Both of these theories are concerned with a deterministic advantage to sex and recombination through the production of genetically variable offspring. This increases efficiency of selection, and hence accelerates the increase in mean fitness. The first of the two theories is known as the Mutational Deterministic Hypothesis (Kondrashov, 1988), and states that sexual reproduction can enable females to reduce the number of deleterious mutations in their offspring. This idea requires that each deleterious mutation leads to a greater decrease in log fitness than the previous mutation (synergistic epistasis between deleterious mutations). The principle theme is that when this is the case, sexual reproduction increases the variance in the number of mutations that will be carried by the offspring. The subsequent lowered fitness of the individuals carrying above average numbers of such deleterious mutations will lead to an increased number of deleterious mutations being eliminated from the population. If the resultant mutation rate per generation is sufficiently high, then this process can theoretically fully compensate for the two-fold cost of sex. However, the genomic mutation rate (U) is exactly where the problem lies, as the plausibility of such a Ratetheory is dependent upon a relatively high rate of mutation within the genome. A female gains the advantage whatever the deleterious mutation rate, but the relative benefit increases with the mutation rate. But what deleterious mutation rate is needed to outweigh the two-fold cost of sex? Kondrashov suggests that the answer depends essentially on the details of the theoretical model, but a rate of about one new deleterious mutation per individual is probably sufficient. Thus, sex becomes advantageous relative to cloning if U is more than about one. This is the most controversial point in this theory, because deleterious mutation rates have historically been thought to be much lower. Mukai has performed a number of experiments on Drosophila and deduced that a mutation rate of 0. 5 per individual per generation was sufficient. The problem concerning mutation rates is difficult to solve as there is no strong factual evidence that exists to rule out mutation rates as high as are required for sex to prosper. However, Mukais estimate of 0. 5 per individual was a lower bound estimate, and his results are also compatible with a figure greater than one. The second of the two modern day models ignores the effect of deleterious mutations and concentrates on external environmental change. This model suggests that sex accelerates adaptation to a changing environment by creating new gene combinations.