A central problem in population genetics is to detect and analyze positive 
natural selection by which beneficial mutations are driven to fixation. The 
hitchhiking effect of a rapidly spreading beneficial mutation, which results in 
local removal of standing genetic variation, allows such an analysis using DNA 
sequence polymorphism. However, the current mathematical theory that predicts 
the pattern of genetic hitchhiking relies on the assumption that a beneficial 
mutation increases to a high frequency in a single random-mating population, 
which is certainly violated in reality. Individuals in natural populations are 
distributed over a geographic space. The spread of a beneficial allele can be 
delayed by limited migration of individuals over the space and its hitchhiking 
effect can also be affected. To study this effect of geographic structure on 
genetic hitchhiking, we analyze a simple model of directional selection in a 
subdivided population. In contrast to previous studies on hitchhiking in 
subdivided populations, we mainly investigate the range of sufficiently high 
migration rates that would homogenize genetic variation at neutral loci. We 
provide a heuristic mathematical analysis that describes how the genealogical 
structure at a neutral locus linked to the locus under selection is expected to 
change in a population divided into two demes. Our results indicate that the 
overall strength of genetic hitchhiking--the degree to which expected 
heterozygosity decreases--is diminished by population subdivision, mainly 
because opportunity for the breakdown of hitchhiking by recombination increases 
as the spread of the beneficial mutation across demes is delayed when migration 
rate is much smaller than the strength of selection. Furthermore, the amount of 
genetic variation after a selective sweep is expected to be unequal over demes: 
a greater reduction in expected heterozygosity occurs in the subpopulation from 
which the beneficial mutation originates than in its neighboring subpopulations. 
This raises a possibility of detecting a "hidden" geographic structure of 
population by carefully analyzing the pattern of a selective sweep.