Nucleotide sequences of mitochondrial DNA (mtDNA) depict the process of molecular evolution and speciation in animals. A dataset of about twenty three thousand sequences of 2 genes, Cyt-b and Co-1, among different species was analyzed at 5 taxa ranks across the Animal Kingdom. The results support the prevalence of a geographic or allopatric speciation and suggest that Darwin's gradual evolution in animals also prevails at the molecular level. The approach suggested allows recognize the

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... c and other speciation modes, using the set theory equations and genetic terms as their components. The suggested approach may solve a key problem of the Biological Species Concept, i.e. the inability of evolutionary studies to monitor the reproductive isolation among species in nature, by defining a species rank with measurable estimates of genetic parameters.