Classically, taxonomists relied on anatomical characteristics of living beings or of extinct fossils for assigning organisms to taxons, so that the classification indicated a hierarchical order of macro-structural complexity. For pre-evolutionary taxonomists, the main task of systematics was (1) to classify, (2) to name, and (3) to indicate degrees of resemblance between organisms. After the theory of evolution was put forward, systematics rapidly became on evolutionary science whose most important aim was to show relationships by descent. Simpson (1944) championed this latter approach to systematics, and moreover, used this classification to estimate the rate of structural evolution of organisms during phylogeny. A good example provided by the lineage of the horse (genus Equus) which has undergone eight successive genera in roughly 45 million years. This works out a approximately 0.18 genera per 106 years, a standard evolutionary rate of organic change which Simpson called “horotelic”. There was also a second approach to the study of organic evolution, based on quantitative characteristics, e.g., brain size, which, as we have previously stated, has increased in the human lineage by a factor of approximately 3 in about 5 million years.
KeywordsAmino Acid Substitution Chromosome Level Pygmy Chimpanzee Eukaryote Genome Nucleotide Replacement
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