Abstract
Selective sweeps reduce neutral genetic diversity. In sexual populations, this "hitchhiking" effect is thought to be limited to the local genomic region of the sweeping allele. While this is true in panmictic populations, we find that in spatially extended populations the combined effects of many unlinked sweeps can affect patterns of ancestry (and therefore neutral genetic diversity) across the whole genome. Even low rates of sweeps can be enough to skew the spatial locations of ancestors such that neutral mutations that occur in an individual living outside a small region in the center of the range have virtually no chance of fixing in the population. The fact that nearly all ancestry rapidly traces back to a small spatial region also means that relatedness between individuals falls off very slowly as a function of the spatial distance between them.
Highlights
In large populations even a fairly low rate of selective sweeps is sufficient to reduce diversity across most of the genome via hitchhiking (Gillespie, 2000; Weissman and Barton, 2012)
We find that probability of fixation of an allele can be strongly position-dependent, with alleles near the center of the range orders of magnitude more likely to fix than those at typical locations. This is because all individuals trace most of their ancestry, even in the not-too-distant past, to individuals living in the center, which causes far-away individuals to be much more closely related to each other than they would be in the absence of the unlinked sweeps, with relatedness falling off only as a power law of distance rather than exponentially
Because selection and demography are often difficult or impossible to measure directly in natural populations, both are typically inferred from patterns of genetic diversity
Summary
In large populations even a fairly low rate of selective sweeps is sufficient to reduce diversity across most of the genome via hitchhiking (Gillespie, 2000; Weissman and Barton, 2012). While the effect of sweeps on genetic diversity at linked loci is reduced by spatial structure, we show here that collective effect of sweeps on the diversity at unlinked loci can be much stronger than in panmictic populations This effect is dependent on the geometry of the range – it only appears for realistic range shapes with relatively well-defined central regions, not for the perfectly symmetric idealizations of ring-shaped and toroidal ranges often used in theoretical models. We find that probability of fixation of an allele can be strongly position-dependent, with alleles near the center of the range orders of magnitude more likely to fix than those at typical locations This is because all individuals trace most of their ancestry, even in the not-too-distant past, to individuals living in the center, which causes far-away individuals to be much more closely related to each other than they would be in the absence of the unlinked sweeps, with relatedness falling off only as a power law of distance rather than exponentially
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