The study describes the use of conserved primers with the polymerase chain reaction (PCR) to amplify and sequence mitochondrial DNA (mtDNA) from over 100 animal species. This method allows for the amplification of mtDNA segments from small, unpurified samples, including museum specimens preserved in alcohol or dry conditions. The sequences obtained show a strong bias toward transitions, particularly in birds and mammals, though thymine to cytosine transitions are less common in birds due to their light strand deficiency. Amino acid replacements in the cytochrome b gene are faster in mammals and birds compared to fishes, aligning with structural hypotheses for cytochrome b. The wide taxonomic utility of these primers offers new opportunities for phylogenetic and population studies. Traditional methods like restriction endonuclease analysis were limited in their ability to assess sequence variation, but PCR provides a faster and more efficient alternative. PCR enables the amplification of unique sequences in hours, with high specificity and automation, making it suitable for large-scale studies. The study also demonstrates that mtDNA differences among species are significant, and that conserved primers can amplify homologous sequences across a wide range of animals, including mammals, birds, fishes, and some invertebrates. The results show that mtDNA sequence evolution exhibits a transition bias, with fewer transversions in birds. The patterns of amino acid replacement fit the structural model of cytochrome b, and the study confirms the reliability of direct sequencing of enzymatically amplified DNA. The findings suggest that mtDNA evolution rates vary among animal groups, with birds and mammals showing higher rates than fishes. The study also highlights the potential of PCR for studying genetic diversity in natural populations, particularly in marine organisms, and for integrating molecular and evolutionary biology to understand genetic structure and function. The research underscores the importance of further studies on substitution matrices and compositional bias in avian mtDNA to better understand deep branches in the avian tree.The study describes the use of conserved primers with the polymerase chain reaction (PCR) to amplify and sequence mitochondrial DNA (mtDNA) from over 100 animal species. This method allows for the amplification of mtDNA segments from small, unpurified samples, including museum specimens preserved in alcohol or dry conditions. The sequences obtained show a strong bias toward transitions, particularly in birds and mammals, though thymine to cytosine transitions are less common in birds due to their light strand deficiency. Amino acid replacements in the cytochrome b gene are faster in mammals and birds compared to fishes, aligning with structural hypotheses for cytochrome b. The wide taxonomic utility of these primers offers new opportunities for phylogenetic and population studies. Traditional methods like restriction endonuclease analysis were limited in their ability to assess sequence variation, but PCR provides a faster and more efficient alternative. PCR enables the amplification of unique sequences in hours, with high specificity and automation, making it suitable for large-scale studies. The study also demonstrates that mtDNA differences among species are significant, and that conserved primers can amplify homologous sequences across a wide range of animals, including mammals, birds, fishes, and some invertebrates. The results show that mtDNA sequence evolution exhibits a transition bias, with fewer transversions in birds. The patterns of amino acid replacement fit the structural model of cytochrome b, and the study confirms the reliability of direct sequencing of enzymatically amplified DNA. The findings suggest that mtDNA evolution rates vary among animal groups, with birds and mammals showing higher rates than fishes. The study also highlights the potential of PCR for studying genetic diversity in natural populations, particularly in marine organisms, and for integrating molecular and evolutionary biology to understand genetic structure and function. The research underscores the importance of further studies on substitution matrices and compositional bias in avian mtDNA to better understand deep branches in the avian tree.