Molecular Anatomy Reveals Evolutionary Relationships

The eighteenth-century naturalist Carolus Linnaeus recognized the anatomic similarities and differences among living organisms and used them to provide a framework for assessing the relatedness of species. Charles Darwin, in the nineteenth century, gave us a unifying hypothesis to explain the phylogeny of modern organisms—the origin of different species from a common ancestor. Biochemical research in the twentieth century revealed the molecular anatomy of cells of different species—the monomeric subunit sequences and the three-dimensional structures of individual nucleic acids and proteins. Biochemists now have an enormously rich and increasing treasury of evidence that can be used to analyze evolutionary relationships and to refine evolutionary theory.

The sequence of the genome (the complete genetic endowment of an organism) has been entirely determined for numerous eubacteria and for several archaebacteria; for the eukaryotic microorganisms Saccharomyces cere-visiae and Plasmodium sp.; for the plants Arabidopsis thaliana and rice; and for the multicellular animals Caenorhabditis elegans (a roundworm), Drosophila melanogaster (the fruit fly), mice, rats, and Homo sapiens (you) (Table 1-4). More sequences are being added to this list regularly. With such sequences in hand, detailed and quantitative comparisons among species can provide deep insight into the evolutionary process. Thus far, the molecular phylogeny derived from gene sequences is consistent with, but in many cases more precise than, the classical phylogeny based on macroscopic structures. Although organisms have continuously diverged at the level of gross anatomy, at the molecular level the basic unity of life is readily apparent; molecular structures and mechanisms are remarkably similar from the simplest to the most complex organisms. These similarities are most easily seen at the level of sequences, either the DNA sequences that encode proteins or the protein sequences themselves.

When two genes share readily detectable sequence similarities (nucleotide sequence in DNA or amino acid sequence in the proteins they encode), their sequences

Carolus Linnaeus, Charles Darwin,

1701-1 778 1809-1882

Carolus Linnaeus, Charles Darwin,

1701-1 778 1809-1882

TABLE 1-4 Some Organisms Whose Genomes Have Been Completely Sequenced

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