Experimental design: Response of a genetically heterogeneous hybrid population to temperature or ethanol selection. S. cerevisiae (CEN.PK) and S. uvarum (CBS7001) were transformed with plasmids conferring G418 and hygromycin resistance, respectively. The two species were mated, then placed under dual antibiotic selection to screen for a viable F1 interspecific hybrid. This F1 was sporulated and the spores allowed to diploidize by mass mating. The resulting genetically mixed hybrid population was used to found 3 experimental populations for temperature selection and 3 experimental populations for ethanol selection. After 500 generations of incremental increases in either temperature or ethanol, three clones were isolated from each experimental vessel and their stress tolerance compared to that of the two ancestral species, an F1 interspecific S. cerevisiae/S. uvarum hybrid, and three isolates from the founding hybrid population.