Mitochondria, semi-autonomous organelles possessing their own genetic system, 
are commonly accepted to descend from free-living eubacteria, namely 
hydrogen-producing alpha-proteobacteria. The progressive loss of genes from the 
primitive eubacterium to the nucleus of the eukaryotic cell is strongly 
justified by the Muller rachet principle, which postulates that asexual genomes, 
like mitochondrial ones, accumulate deleterious and sublethal mutations faster 
than sexual genomes, like the nucleus. According to this principle, the 
mitochondrial genome would be doomed to death; instead, we observe that the 
mitochondrial genome has a variable size and structure in the different 
organisms, though it contains more or less the same set of genes. This is an 
example of genetic conservation versus structural diversity. From an 
evolutionary point of view the genetic system of organelles is clearly under 
strong selective pressure and for its survival it needs to utilize strategies to 
slow down or halt the ratchet. Anyway, the mitochondrial genome changes with 
time, and the rate of evolution is different for both diverse regions of the 
mtDNA and between lineages, as demonstrated in the case of mammalian mt genomes. 
We report here our data on the evolution of the mitochondrial DNA in mammals 
which demonstrate the suitability of mtDNA as a molecular tool for evolutionary 
analyses.