Abstract
A steady influx of a single deleterious multilocus genotype will impose genetic load on the resident population and leave multiple descendants carrying various numbers of the foreign alleles. Provided that the foreign types are rare at equilibrium, and all immigrant genes are eventually eliminated by selection, the population structure can be inferred explicitly from the branching process taking place within a single immigrant lineage. Unless the migration and recombination rates were high, this novel method gives a close approximation to the simulation with all possible multilocus genotypes considered. Once the load and the foreign genotypes frequencies are known, it becomes possible to estimate selection acting on the invading modifiers of (i) dominance and (ii) recombination rate on the foreign gene block. We found that the modifiers of the (i) type are able to invade faster than the type (ii) modifier, however, this result only applies in the strong selection/low migration/low recombination scenario. Varying the number of genes in the immigrant genotype can have a non-monotonic effect on the migration load and the modifier’s invasion rate: although blocks carrying more genes can give rise to longer lineages, they also experience stronger selection pressure. The heaviest load is therefore imposed by the genotypes carrying moderate numbers of genes.
Highlights
Gene flow is a major force shaping the evolution of closely related sexual populations, which can prevent local adaptation [1,2] or facilitate generalism [3,4], erase or maintain intraspecific polymorphism [5,6], promote the evolution of female mating preferences [7,8] and lead to speciation through reinforcement [9,10]
This measure turns out to be equal to the actual migration load imposed on the population by gene flow and calculated at any time point once the migration-selection process is at equilibrium, because immigrant lineages, arriving at a slow rate, will segregate independently in a large resident population
The population at equilibPrium is characterized by the mean population fitness w~1{mz piwi, i estimated at the haploid stage, where 12m is the resident genotype frequency after migration, and pi is the frequency of the foreign genotype i weighted by its relative fitness wi
Summary
Gene flow is a major force shaping the evolution of closely related sexual populations, which can prevent local adaptation [1,2] or facilitate generalism [3,4], erase or maintain intraspecific polymorphism [5,6], promote the evolution of female mating preferences [7,8] and lead to speciation through reinforcement [9,10]. More rigorous theoretical treatment of the introgression, is, difficult since too many genotypes have to be considered even for a moderate number of loci: significant progress in this area has been made but involves a great deal of approximation and simplifying assumptions about the nature of recombination of the foreign set of genes [17,18] We develop this theory further and propose a new simple model of gene flow and selection that allows for an explicit characterization of the resident population structure, provided that the population is large, migration rate is low and the foreign genes are evenly distributed in a linear genome block
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