Mitochondrial DNA: more than an evolutionary bystander

Summary 1. The vast majority of studies employing mtDNA in evolutionary biology and ecology have used it as a means to infer demographic and historical patterns without pondering the underlying functional implications. In contrast, the biochemical and medical communities often aim to understand the influence of specific mtDNA mutations on mitochondrial functions, but rarely consider the evolutionary and ecological implications. 2. Ongoing research has shown that mtDNA mutations can profoundly affect mitochondrial function in humans and other animals. If the mutation (or set of mutations) is pathogenic, mitochondrial malfunction may be detected from early age. In nature, however, most mutations are not highly deleterious and may exist at intermediate frequency in populations. 3. In this review, we suggest that knowledge of the underlying biochemistry and functions of mitochondria can facilitate a more complete determination of the evolutionary dynamics of mtDNA and its influence on the life-history traits of organisms. With this approach, it is possible to use biochemistry to link the genotype with the phenotype. 4. After reviewing the literature, we conclude that there can be physiological and evolutionary trade-offs in the way that mitochondrial mutations can affect age classes and/or fitness components and that these effects may depend on the environment. Through these trade-offs, it may be possible for specific mtDNA mutations to have unequal fitness in different nuclear genetic backgrounds and also in different environments.