Mitochondrial DNA variation within natural populations of D. subobscura is heavily dominated by the two haplotype groups HI (blue) and HII (red) in all three disjunct parts of its global distribution (white). Apart from populations on the Canary Islands, where other haplotypes (green) are common, the mtDNA haplotype frequencies are similar across populations, with on average 37% of all individuals carrying HI and 58% HII haplotypes. This observation alone suggests that balancing selection is contributing to the maintenance of genetic variation in mtDNA. Several experimental studies have shown that flies carrying HI and HII differ in key life history traits, such as as metabolic rate, development time, adult longevity and desiccation resistance. Distribution map reproduced with permission from Rodríguez-Trelles et al.