Organismal phenotypes provide the characters for phylogeny reconstruction. In the first 100 years following Darwin, scientists estimated phylogenetic trees for various taxa by comparing visible organismal phenotypes – e.g., morphological, physiological, or behavioral characteristics – that they could only presume must reflect the true evolutionary genetic relationships of organisms.
phylogeny; organismal relationships; cytochrome c
Organismal phenotypes provide the characters for phylogeny reconstruction. In the first 100 years following Darwin, scientists estimated phylogenetic trees for various taxa by comparing visible organismal phenotypes – e.g., morphological, physiological, or behavioral characteristics – that they could only presume must reflect the true evolutionary genetic relationships of organisms.
In the 1950s and 1960s, several molecular technologies were developed that eventually would give direct and quick access to voluminous genetic information recorded in DNA and protein sequences. In 1963, Emanuel Margoliash introduced the revolutionary idea that organismal relationships could be deduced from such macromolecular data. By compiling published information on the molecular strings (each 104 amino acids long) of the cytochrome c protein from several sources – human, pig, horse, rabbit, chicken, tuna, and yeast – Margoliash concluded, “the extent of variation among cytochromes c is compatible with the known phylogenetic relations of species. Relatively closely related species show few differences…phylogenetically distant species exhibit wider dissimilarities.” In 1967, Fitch and Margoliash pioneered a formal phylogenetic procedure for constructing evolutionary trees from such molecular sequences. More recently, molecular phylogenetics has become a flourishing enterprise with many available algorithms suited for different categories of genetic data.
Organismal-level phenotypes continue to be a useful and ready source of phylogenetic information from many taxa, but generally they have been superseded by molecular data for detailed phylogenetic appraisals. Molecular characters are of special significance because they are: (1) unambiguously genetic; (2) ubiquitously distributed across all forms of life (including microbial); (3) quantifiable and copious in number; (4) arguably less prone to homoplasy (evolutionary convergences or reversals) that otherwise can complicate phylogeny estimation from adaptive phenotypic traits; and (5) arguably less variable in their evolutionary rates (see Chapter 34). Today, molecular phylogenetics is a robust and multifaceted discipline, with molecular phylogenies routinely being published for a wide diversity of taxa. Indeed, it is not hard to imagine a time in the not-too-distant future when the success of molecular phylogenetics leads to the field’s inevitable demise, because molecular phylogeneticists will have reconstructed (about as well as is theoretically possible) essentially all major branches and twigs in the Earth’s tree of life. Thus, this successful enterprise will have put itself out of business.
1. Margoliash E. Primary structure and evolution of cytochrome c. Proc Natl Acad Sci USA. 1963;50:672–679.
2. Fitch WM, Margoliash E. Construction of phylogenetic trees. Science. 1967;155:279–284.
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