The fixation probability of rare mutators in finite asexual populations

A mutator is an allele that increases the mutation rate throughout the genome by disrupting some aspect of DNA replication or repair. Mutators that increase the mutation rate by the order of 100 fold have been observed to spontaneously emerge and achieve high frequencies in natural populations and in long-term laboratory evolution experiments with \textit{E. coli}. In principle, the fixation of mutator alleles is limited by (i) competition with mutations in wild-type backgrounds, (ii) additional deleterious mutational load, and (iii) random genetic drift. Using a multiple locus model and employing both simulation and analytic methods, we investigate the effects of these three factors on the fixation probability $P_{fix}$ of an initially rare mutator as a function of population size $N$, beneficial and deleterious mutation rates, and the strength of mutations $s$. Our diffusion based approximation for $P_{fix}$ successfully captures effects (ii) and (iii) when selection is fast compared to mutation ($\mu/s \ll 1$). This enables us to predict the conditions under which mutators will be evolutionarily favored. Surprisingly, our simulations show that effect (i) is typically small for strong-effect mutators. Our results agree semi-quantitatively with existing laboratory evolution experiments and suggest future experimental directions.Comment: 46 pages, 8 figure

The transnational migration strategies of Chinese and Indian students in Australia.

Migration is increasingly transient, particularly among the highly skilled. In addition, countries engaged in a race for highly skilled labour are prepared to modify immigration regulations to attract those migrants. Australian immigration policy reflects how Australia sees international students as highly skilled migrants. Despite abundant research regarding the mobility of the highly skilled, there is a relative lack of investigation into the mobility of international students and their subsequent migration patterns. This thesis explores the nexus between immigration policy and international education by investigating the determinants of the mobility of Chinese and Indian students in Australia. Chinese and Indian students enrolled in the University of Adelaide and the University of South Australia were surveyed and key informants were interviewed to uncover the underlying factors that motivated the students in going abroad to study and their future intentions when they graduate. It was found that while decisions were based on myriad factors, Chinese and Indian students were motivated differently. Findings revealed Permanent Residency in Australia as a key objective for many; with Indian students more driven than their Chinese counterparts in seeking this outcome. Skilled migration programmes designed to encourage the permanent settlement of international students influenced the deliberate selection of Australia and Adelaide as study destinations; however, questions of permanency were raised as Chinese and Indian students, particularly the latter, exhibited post study migration intentions to either interstate and/or third country destinations. The limited effectiveness of policy was highlighted as Australia's skilled migration programme was circumvented through the use of Australia and/or Adelaide as gateways to preferred destinations. In other words, as much as policy can influence the decision of students when deciding on study destinations, students also subvert policy by configuring particular study destinations as gateways en route to a preferred destination. The implications for policy and its role in shaping the migration strategies of the students are thus investigated. International student mobility is conceptualised in this thesis as linkages to permanent settlement and onward migration highlight the inadequate and limited scope of traditional forms of migration research in explaining student migration.Thesis (Ph.D.) -- University of Adelaide, School of Social Sciences, 2012

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