On the evolutionary effects of linkage and deleterious mutations in small populations

Sammanfattning: The aim of this thesis is to evaluate the effect of deleterious mutations and linkage on variation in neutral markers and fitness in small populations. The study is based on two main approaches, one by using computer simulations, the other by exploring genetic variation in a cyclical parthenogen, Daphnia pulex (Crustacea: Cladocera).Deleterious mutations can lead to an associative overdominance at a linked neutral marker in small populations which result in a positive relationship between heterozygosity and fitness. Such an effect can arise due to marginal effects of selection on multiple loci. The effect of deleterious mutations can both reduce and increase heterozygosity at a linked neutral marker, depending on parameters of selection, dominance, recombination rate and population size. The heterozygosity reducing effect referred to as background selection becomes dominant with increasing population size and stronger selection. The mode of reproduction affects the linkage among loci. Estimates of population subdivision, based on neutral markers, are dependent on whether the effect of deleterious mutations increase or decrease variation at the neutral marker. In small populations the effect can lead to underestimates of subdivision.Associative overdominance arises more easily in cyclical parthenogenetic populations, where reproduction alternates between sexual and clonal periods, than in sexual populations. The effect of deleterious mutations is dependent on the length of the asexual period. A long period can result in associative overdominance at a neutral marker and in a large segregational load, observed as a drop in fitness among sexually produced individuals.Microsatellite variation in Daphnia pulex from Sweden and Finland revealed two different evolutionary lineages, characterised with high level of subdivision and isolation by distance. The variation indicates a non-equilibrium system, shaped possibly by fluctuations in population size or selection.Variation in fitness related traits, asexual propagation and sexual response, in Daphnia pulex are dependent on crowding, and has a genetic basis.