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A whole-genome duplication occurred in a shared ancestor of the yeast species Saccharomyces cerevisiae, Saccharomyces castellii and Candida glabrata. Here we trace the subsequent losses of duplicated genes, and show that the pattern of loss differs among the three species at 20% of all loci. For example, several transcription factor genes, including STE12, TEC1, TUP1 and MCM1, are single-copy in S. cerevisiae but are retained in duplicate in S. castellii and C. glabrata. At many loci, different species have lost different members of a duplicated gene pair, so that 4–7% of single-copy genes compared between any two species are not orthologues. This pattern of gene loss provides strong evidence for speciation through a version of the Bateson–Dobzhansky–Muller mechanism, in which the loss of alternative copies of duplicated genes leads to reproductive isolation^,. We show that the lineages leading to the …