Influence Of Factors On The Adaptation Rate And The Evolution Of Sex

Evolution is the changing process of genetic components of a population. The purpose ofpopulation genetics is to study the population genetic structure or evolutionary law, andfurther to elucidate the evolutionary mechanism. Despite previous efforts on evolutionarybiology, many basic questions in evolution area are unsolevd because of the limitation ofexperimental condition and time-consuming. As the development of modern computertechnology, simulations provide a more extensive way to investigate these evolutionaryquestions. In the present doctoral dissertation, I adopt computer simulations to study thefactors that are important to influence evolutionary process. The current research includesthree parts as follows, from the simple asexual populations to complex sexual populations.Part1: impacts of mutation effects and population size on mutation rate in asexualpopulations. Mutation is the mainly source of genetic variation in natural population thatrequired for evolutionary adaptation and novelty. However, most those mutations withphenotypic effects are especially harmful and are consequently removed by natural selection.For this reason, an organism will evolve to a lower mutation rate. The action of naturalselection on mutation rate is related to population size and mutation effects. Althoughtheoretical work has intensively investigated the relationship between natural selection andmutation rate, most of these studies have focused on individual competition within apopulation, rather than on competition among populations. The aim of the present study wasto use computer simulations to investigate how natural selection adjusts mutation rate amongasexually reproducing subpopulations with different mutation rates. The competition resultsshowed that a population could evolve to an “optimum” mutation rate, and that this rate wasmodulated by both population size and mutation effects. A larger population could evolve to ahigher optimum mutation rate. The optimum mutation rate increased with both the fractionand the effects of beneficial mutations, rather than with the effects of deleterious mutations.When strongly favored mutations appeared, the optimum mutation rate was elevated to amuch higher level. The competition time among the subpopulations also substantiallyshortened. Beneficial mutations were the leading force that modulated the optimum mutation rate. The initial configuration of the population appeared to have no effect on theseconclusions, confirming the robustness of the simulation method developed in the presentstudy. These findings might further explain the lower mutation rates observed in most asexualorganisms, as well as the higher mutation rates in some viruses.Part2: the influence of deleterious mutations on adaptation in asexual populations.I study the dynamics of adaptation in asexual populations that undergo both beneficial anddeleterious mutations. In particular, how the deleterious mutations affect the fixation ofbeneficial mutations was investigated. The results show that in the “strong-selection weakmutation (SSWM)” regime or in the “clonal interference (CI)” regime, deleterious mutationsrarely influence the distribution of “selection coefficients of the fixed mutations (SCFM)”. Bycontrast, in the “multiple mutations” regime, the accumulation of deleterious mutations wouldlead to a decrease in fitness evidently. Therefore, the effects of deleterious mutations onadaptation depend largely on the supply of beneficial mutations. Interestingly, the lowestadaptation rate occurs for a moderate value of selection coefficient of deleterious mutations.Part3: the relative effects of segregation and recombination on the evolution of sexin finite diploid populations. The mechanism of reproducing more viable offspring inresponse to selection is considered as a major factor influencing the advantages of sex. Indiploids, sexual reproduction combines genotype by recombination and segregation.Theoretical studies of sexual reproduction have investigated the advantage of recombinationin haploids. However, the potential advantage of segregation in diploids is less studied. Thepresent study aimed to quantify the relative contribution of recombination and segregation tothe evolution of sex in finite diploids by using multi-locus simulations. The model calculatedthe mean fitness of a sexually or asexually reproduced offspring to describe the long-termeffects of sex. The evolutionary fate of a sex or recombination modifier was also monitored toinvestigate the short-term effects of sex. We considered two different scenarios of mutationsin finite populations to investigate the evolution of a sex or recombination modifier:(1) onlydeleterious mutations were present and (2) a combination of deleterious and beneficialmutations. Results showed that the combined segregation and recombination stronglycontributed to the evolution of sex in diploids. If deleterious mutations were only present,segregation efficiently slowed down the speed of Muller’s ratchet. As the recombination levelwas increased, the accumulation of deleterious mutations was totally inhibited andsubstantially contributed to the evolution of sex. The presence of beneficial mutationsevidently increased the fixation rate of a recombination modifier. We also observed that thetwofold cost of sex was easily to overcome in diploids if a sex modifier caused a moderate frequency of sex.The current study associated new computer simulations, along with mathematical modeland statistic analysis, to investigate the factors that influence adaptation rate. I havesystematically illustrated the following issues:1), how population size and mutaion effectsinfluence the optimum mutation rate in asexuals?2), how deleterious mutations influence thelong-term effects of adaptation rate in asexuals?3), the relative advantage of segregation andrecombination to the evolution of sex in diploids. Our methods provide new insights to therelated question of evolution.