Backmapping the phylogenetic tree for an organism's evolutionary pathway is the science of systematics. Systematics establishes degrees of kinship, diversity, evolutionary linkages, as well as taxonomic classification for all species.
A common method of establishing kinship is the identification of homologous structures shared by two different species.
For instance, a human arm, an alligator foreleg, and a penguin's flipper are homologous structures not because they look alike or function similarly, but because they both share a common bone structure indicating they all evolved from the same bone feature. The more homologous structures in common, the greater the degree of kinship.
A homologous structure is an anatomical term referring to commonalities in structural characteristics that evolved from the same feature in an ancestor.
Analogous structures are often confused with homologous structures because they appear to be similar in structure and function and resulting behavior. Convergent evolution occurs when dissimilar species living in the same territory and under the same environmental pressures develop analogous or similar structures that function in response to natural selection. As an example, both sharks and whales share common body structures that allow them to swim in the water. However, these structures are analogous, not homologous, because they trace their origin to different features: Whales are mammals, and sharks are fish! An embryonic analysis is often used to confirm homologous or analogous structures. Analogous structures do not indicate a degree of kinship.
Anatomical analyses, such a homologous structures, have been the foundation of systematics until the onset of more sophisticated molecular analysis. Because this emerging technology has allowed for the identification and sequencing of proteins and nucleic acids, each organism can be individualized.
The comparison of DNA nucleotide sequences between two separate specimens is the most accurate method of establishing kinship. Other methods include an analysis of rRNA nucleotide sequences and the extent of hydrogen-bonding similarities between DNA strands.
Similarly, a comparison of the sequencing of amino acids in a polypeptide provides the most accurate analysis of protein structures. This analysis of protein backmaps to the DNA that coded for the amino acid sequence. The closer the match between amino acid segments, the more similar the DNA, the closer the phylogenetic relationship.
Excerpted from The Complete Idiot's Guide to Biology © 2004 by Glen E. Moulton, Ed.D.. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.