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Introduction to Biological Classification

Author(s): Deanne Erdmann, MS

Systematics: Evolutionary Classification of Organisms

Evolutionary classification, or phylogenetics, creates classification that represents hypothesized relationships among groups of organisms. Systematists use a combination of fossil records, comparative anatomy, cladistical analyses and molecular data to understand the patterns of relationships among organisms. 

The fossil record is an accumulation of all fossils found within layers of sedimentary rock and helps to reconstruct a geological time scale. Fossils are the remnants or impressions of organisms that lived in the past. 

Homologies are similarities among species attributed to the inheritance of a feature from a common ancestor.  Important information about common ancestry can be discovered by comparing different organisms' anatomical, embryological and molecular homologies. A classic example of homologous structures is the comparison of the basic groups of bones in the forelimbs of different groups of vertebrates (whale, alligator, penguin and human). Although each forelimb is adapted for a different use, the bones are formed in the same way during embryological development, suggesting descent from a common ancestor. 

Cladistics is based on the idea that members of a group share a common evolutionary history and are more closely related to members within their group than to other organisms. These groups are recognized as sharing unique, derived features not present in distant ancestors. A cladogram is a branching diagram that illustrates hypothesized relationships based on shared, derived characteristics.

Comparative sequencing: Scientists also can compare DNA and RNA sequences among different organisms to unravel evolutionary relationships and common ancestry. These sequences can be used in comparison studies to determine phylogenetic relationships that can not be compared between morphological or fossil data. Ribosomal RNA, chloroplast DNA, and mitochondrial DNA have proven particularly useful in these kinds of studies.

Molecular clock studies compare sequences of macromolecules (proteins and nucleic acids) among species, assuming that these macromolecules evolve at constant rates throughout time, and for different lineages. Changes in sequences (nucleotide or amino acid substitutions, or mutations) are used to develop ideas about the evolutionary divergence of species. The molecular clock hypothesis has been a powerful technique for determining evolutionary events of the remote past for which the fossil record and other evidence is lacking or insufficient. The reliability of this hypothesis is currently under debate in the scientific community.