Outcrossed sex exposes genes to competition with their homologues, allowing alleles that transmit more often than their competitors to spread despite organismal fitness costs. Mitochondrial populations in species with biparental inheritance are thought to be especially susceptible to such cheaters because they lack strict transmission rules like meiosis or maternal inheritance. Yet the interaction between mutation and natural selection in the evolution of cheating mitochondrial genomes has not been tested experimentally. Using yeast experimental populations, we show that although cheaters were rare in a large sample of spontaneous respiratory-deficient mitochondrial mutations (petites), cheaters evolve under experimentally enforced outcrossing even when mutation supply and selection are restricted by repeatedly bottlenecking populations.