Mitochondria are small organelles that hold big responsibility in the cells of all eukaryotes -- they provide ~80-90% of the energy. It is thought that mitochondria are remnants of symbiotic or enslavement (still under debate) of purple non-sulfur bacteria ~1.4 billion years ago. This is why mitochondria have their own mitochondrial genomes, in addition to the nuclear genomes in the nucleus.

For a long time, it was thought that due to the importance of the role of mitochondria for the cell, and since their genomes are separate from the nuclear genome among other reasons, that mitochondrial genomes evolved neutrally. That is, mitochondrial genomes did not evolve to adapt to new environments or functions.

However, more and more studies are now showing that there is evidence of evolution of mitochondrial genomes in systems where energy requirement is increased. This includes our work where we tested this hypothesis by using two completely separate datasets of electric fishes.