Little information is presently available on the factors promoting genetic divergence in eukaryotic microbes. We studied the spatial distribution of genetic variation in Saccharomyces paradoxus , the wild relative of Saccharomyces cerevisiae , from the scale of a few centimetres on individual oak trees to thousands of kilometres across different continents. Genealogical analysis of six loci shows that isolates from Europe form a single recombining population, and within this population genetic differentiation increases with physical distance. Between different continents, strains are more divergent and genealogically independent, indicating well-differentiated lineages that may be in the process of speciation. Such replicated populations will be useful for studies in population genomics.