Gene redundancy has been observed in yeast, plant and human genomes, and is 
thought to be a consequence of whole-genome duplications. Baker's yeast, 
Saccharomyces cerevisiae, contains several hundred duplicated genes. 
Duplication(s) could have occurred before or after a given speciation. To 
understand the evolution of the yeast genome, we analysed orthologues of some of 
these genes in several related yeast species. On the basis of the inferred 
phylogeny of each set of genes, we were able to deduce whether the gene 
duplicated and/or specialized before or after the divergence of two yeast 
lineages. Here we show that the gene duplications might have occurred as a 
single event, and that it probably took place before the Saccharomyces and 
Kluyveromyces lineages diverged from each other. Further evolution of each 
duplicated gene pair-such as specialization or differentiation of the two 
copies, or deletion of a single copy--has taken place independently throughout 
the evolution of these species.